Before the main attack on Minas Tirith came, Osgiliath was attacked. Amphibious attacks were launched across the river in conjunction with Ring Wraith over flights. These defeated the local garrison. Hastily constructed bridges across the river provided a means to get supplies and the siege equipment across before a Gondorian counterattack.
The Conflict
Prelude
Now, remember the 28 divisions Mordor is supposed to have and 6 kilometers that a division is expected to cover in Deep Battle Doctrine? That means the army of Mordor would be attacking along a front 168 kilometers (104.4 miles) wide against a 5,000 man Gondorian army. To give you an idea here, that smaller distance is the distance between Minas Morgul and Minas Tirith. Basically, a deep battle style envelopment of Gondor’s military seems very easy to do. The 25 kilometer distance deep battle calls for would be in easy reach for an orc army able to cover 165 miles in 60 hours with little rest. This is 4.4 kilometers per hour or 80 a day assuming only 18 hours marching. Given how it looks like Minas Tirith is only 50 miles from Minas Morgul, the entire army would get there in less than a day if no resistance is encountered.
But we have to remember something important. This entire army would have to cram through the narrow pass we see Frodo, Sam and Gollum squeeze through. A German corps had a total of 30,000 men. The entire length of this formation in a two lane road would be 29 kilometers (18 miles) long – out of a 32 kilometer (20 mile) maximum march distance. That means this army would stretch out for 251.62 kilometers and take at least a day to march through the pass, reform then attack. It would take 4 to 8 days to form the attack if it was to have its full power. If Sauron delays attack until all of his forces are outside Mordor and are able to establish a solid formation, he gives up more and more surprise. But if units strike out instantly after they leave Minas Morgul, they lose a lot of power. In fact, the army increases the risk of defeat in detail the more Sauron adheres to this plan.
One could argue that this can be avoided by sending part of the army through the Black Gate. But, this would cause conflict with the Easterlings. The size of the Easterling army is already so large that it would be suffering from logistics and space problems without any additional orcs clogging the roads. Moving south would require the army moving hundreds of miles around the mountains forming Mordor’s borders. Then, they would get in the way of the Haradrim and Corsairs. Plus, any diversion of troops from the central thrust risks it losing vital power. The only solid option is the Minas Morgul option.
Basically, this would be a perfect bottleneck for a Thermopylae style blocking action. This also shows the value of the trolls, who would be quite effective in eliminating this impasse. Also possible would be a surprise attack by either rangers or cataphracts/knights. This would be highly disruptive in this forming up stage. Probably would not effectively stop the attack, but it would do more damage than at any other point. It would also be more of a delay than any other point. Once it forms up though, the siege of Minas Tirith becomes inevitable.
One of the main areas in which Mordor's attack would be premature is in the time of year. First, the eagles would be more active. Their preference is to strike out and eliminate threats before they lay their eggs so they can care for their young in peace. Helm's Deep and the destruction of Isengard both took place at the very beginning for bald eagle mating seasons in Alaska or a few months before for more southern eagles. Which means a bit of a delay on Mordor's part would keep the eagles out more.
The other extremely important element is the fact Gondor's Compagnie d'ordonnance troops would be available in much larger numbers. If Mordor had waited until the dead of winter, it would essentially have just Gondor's professional troops to deal with. Now, the full might of Gondor can be mustered. This at first would materialize just in the regions of the Southern Fiefs. But these troops would move up as Aragorn does.
So, given the fighting styles of the Elves, I expect their trainers and military advisers to primarily be located along the border with Mordor. They would be openly deployed so they would be effective as a tripwire force. Through their help, the Gondorian Rangers would have been expanded. I would suspect not by too much though. It is doubtful that there would be the people for it. Maybe not because of low population. These men operate in the most dangerous environments and far outside Gondor's formal command and control. So the need for demonstrated higher skill, experience and loyalty would probably be the main limiting factor. So the bump would be to 2 banda total. Which means I suspect that the force countering Mordor's initial attack would be a force of 400 Rangers. The elves would probably have 600 troops in the area. That way we can effectively get a solid Tagma in the area as the first line of defense.
Then there are the standard professional troops. Due to the legitimate fears of exactly this sort of assault occurring, I posited that two Tagma would be stationed in the area of Osgiliath. This means 2,000 regular soldiers. The 48 urban cohorts would be present as well. But these would primarily retreat to the city itself instead of participating in the opening clashes. The Cataphracts would be the most useful and rapid in their deployment, so they might participate in the fighting prior to Mordor reaching the city gates. But the infantry would need all the time these opening battles allow them to get to Osgiliath.
So, if we remember the numbers from my breakdown of Mordor's military, I mentioned it had between 140,200 and 260,300 orcs in 9,380 strong divisions. This results in between 15 and 28 divisions of orcs. Each division would need 238 trolls. This to me is where the actions of Isengard would most directly screw Mordor over. Sauron would naturally prefer to get more trolls, but that probably takes a lot of time. So I'm going to say Mordor has 15 Assault Divisions with the full amount, equaling 140, 200 orcs and 3,570 trolls. The remaining 120,100 orcs would be in 13 regular divisions with no trolls.
The outer wall around Minas Tirith was able to be rebuilt relatively quickly. So we can assume from this two important things. First, retaking the Eastern side should have allowed for the outer defenses for that side to be made battle ready. Probably not as good as they would have been in the golden era of the city, but certainly serviceable. I'm merely expecting the repairing of the major combat damage in gatehouses and important sectors of wall.
The second is that at a minimum, there should have been some sort of prepared defenses along the river to block a crossing. Seems like this would be the easiest, cheapest and quickest thing they could have done. Plus, solid prepared defenses have a clear track record of maximizing the power of defenders.
We also know that a general that attempts to use one burned out bridge to secure the far side of a river would be an idiot. Especially with the numbers at hand. Getting siege engines does require more impressive bridges, but they are much easier to block by defenders. Instead, sending infantry across on boats or over the much easier/quicker to build pontoon boats can secure the other side over a wider front. This buys the attackers space to construct the more impressive bridges needed to move their equipment.
All this to say that it is so much easier to do some sort of earthwork on the western bank of the river over which Rangers can fire. This would allow the entire river to be defended and is closer to the whole doctrine I think Gondor would use - many different fortified fall back positions. Then there is the fact than anyone can tell you that how Mordor attacked in the movies would be how they would attack and Gondor had the time to prepare appropriate positions. So there is no excuse for there to be nothing like this.
The first medieval wall around Paris was similar to this actually. It had a 12 by 3 meter wide ditch with a likely wood fence. I can easily see this being the outside the main external wall, similar to how the Byzantium walls were set up. This would also be effective as an improvised defense along the river bank. We know from Roman days up to World War One that motivated infantry with shovels can construct similar defenses with relative ease. The riverbank version would likely be reinforced with debris from buildings on the bank destroyed intentionally to make room for this wall.
So, by this point the rangers and Elves would have been forced back. This would force Rivendell into the war and draw Lothlorien down too. Mirkwood would be too caught up in the northern battle to have many troops to spare. So At least 2,400 from each of those realms would be on the move because of the events thus far. But their arrival would not come until later.
Given the incredible disparity in numbers, there would be a two (technically three) pronged attack plan. The first would be brute force across the river. This would be done primarily with the normal divisions. Trolls would be extremely difficult to move in secret or under enemy fire via boat against entrenched positions. But normal orcs would not suffer from this limitation. They would be able to move in relative silence at night similar to how they did in the movies. Then it is just a matter of fighting across those prepared positions on the Gondorian side. They will take a a lot of losses. But they will break through.
The other two prongs would be north and south of the city. This is where the assault divisions would come in handy. Their goal would be to encircle the city and cut it off from all aid. Then, some would charge as fast as possible to Minas Tirith. The goal would be to cut off any retreating enemies or reinforcements from other areas. The ones staying in Osgiliath would break through relatively easily. After all, the outnumbered garrison would already have its hands full defending the river. This would not be too hard to pull off. The sheer numbers of this attacking force makes going around the city almost impossible not to do to some degree.
At this point, the standard rank and file troops would race Mordor to Minas Tirith. Essentially, them remaining in the city would result in more harm than good. First off, that many men would result in starvation at best. More likely they would all get killed. The numbers against them would be so high that there would essentially be no hope for them winning in a straight pitched battle. Especially with the likelihood of the river defenses getting breached. Instead they would head to Minas Tirith, engaging in essentially a fighting retreat and delaying tactics. Ideally, they would slow down Mordor's forces and inflict some losses on them. But getting to Minas Tirith while minimizing losses would be the prime objective.
The Rangers, Elves and Urban Cohorts would be much better suited for the sort of brutal urban fighting that would result. More important is the fact that they would be a much smaller force. Which means that they can shorten their lines. So they can handle the shorter sections of walls closer to the bridge. The two main buildings on the bridge should have enough space to house all of them and some supplies. Especially given the fact that elves would be part of the force present. The Lembas bread would be perfect supplies for a commando force on the move like they were before the siege. This would also be the ideal food for those trapped in a siege.
The result would be the garrison retreating to the bridge and attempting to hold out. This is not that bad of an idea. First, the encirclement means leaving the city would be suicidal. Second, this bridge is surprisingly well protected with many layers of strong defenses on its own. Especially when one looks at the fact it is a bridge. Then, there is the whole reinforcement potential. The bridge access from the river allows for supplies and reinforcements to get in or an emergency escape if needed.
Now, getting men in and out now would be highly risky. The Ring Wraiths would control the skies and orcs control the banks. Easterlings would be marching from the north and the Corsairs would be coming from the south. So there is not a whole lot of routes one can take to get in safely. But it is possible.
Now, I mentioned elsewhere that the attack forces would leave from Minas Morgul and be able to get to Osgiliath in roughly 5 days. The march of the Easterlings from the Black Gate would take a minimum of 10. Which means that Osgiliath would be mostly taken before they get in the area. This force would be 6 legions and 8 auxilia brigades. I think this force would primarly be sent to the main action at Minas Tirith.
However, I think two brigades would be left behind - one on each side of the bridge. Since the combined force would be 7,680, the odds would be in their favor. After all, this is bigger than Gondor's professional army on its own. This does not factor in the orcs present either. Though this would be seen as a mopping operation. Because the garrison would be seen as mostly already defeated, the legions would move on to where the glory is to be found.
At this point this is where the clock sets in. If we keep with the time constraints of the books, we have 15 days this garrison needs to last. At that point, the One Ring gets destroyed. So like Minas Tirith, the battle ends regardless of Mordor's army's fighting ability. Then, before that happens the reinforcements would arrive. Aragorn would have the forces from the Southern Fiefs, the Grey Company and the Army of the Dead. From the north would be the Rohirrim, elves and dwarves. Possibly the ents too. The Eagles would naturally be covering the river as this is where the targets most vulnerable to attack from the air would be. So it seems entirely possible that this would be where the battle ends.
Effectively, the siege would be broken somehow. It is difficult to tell who would do it though. Aragorn would have less on his plate. He would just need to get the Army of the Dead and defeat the Corsairs/Southrons. But he would also be struggling with getting the Southern Fiefs on board. Their troops would need to march to the ships. Of course after their lords are convinced that the threat is removed. There would be politics getting in the way of ideal speeds. Those in the north have much further to go and have battles at Dol Guldor and Cair Andros to worry about. But once that is taken care of, they too can use the same river to move south.
One group of reinforcements would probably make it straight for Minas Tirith. That being the troops sent or forced away from Cair Andros. They would not have what it takes to move down the river under enemy fire. Plus, they too would probably feel that battle is lost.
The Rohirrim though could theoretically do it. Their mounted force would be much faster. They used a secret path to get to the area and bypass enemy scouts and defenses. Then, the wide open plains between the two cities would probably be perfect ground to charge through. But the city walls of Osgiliath would make it hard for them to get in. The beseiging army would naturally exploit them to cover their rear areas as they attack the bridge holdout. Plus, urban warfare is not ideal for massed heavy infantry. So they would probably do massive damage but be forced to turn back to Minas Tirith. Which means the other two armies remain.
Now, it would be tempting to say that Osgiliath would turn into another Cair Andros. After all, it too would control the same river as Pelegir and Cair Andros. Essentially, it is where all three attacking armies would theoretically meet. But it would be a relatively simple matter to go around in the case of Mordor - as already discussed. The other armies would just need to get off a bit sooner. Which is much easier with Mordor already controlling the territory in question. So the need to totally take the city is not needed. Once the garrison is trapped in the bridge and the rest of the army is able to move on, Mordor got what it needed. Yes, Mordor will be defeated. But it can lose this battle without that hampering the ability to totally defeat Gondor.
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