Magical Communication, Intelligence, Reconnaissance and Surveillance

Well before the Great War, all respectable armies used various magical means to gather information on enemy deployments and movements as well as communicate to coordinate manoeuvres.
Locate Creature (4th) and Locate Animals (2nd) only work at sort range, and were therefore only applicable for tactical situations. While Scrying (5th) has no range limit, it short duration seriously limits its applicability. Familiars were generally the best option for reconnaissance. In particular, Warlocks with the Voice of the Chain Master Evocation, with their unlimited communication range were highly sought after by military organization.
As countermeasures, military organizations would deploy trained dogs, particularly from breeds with keen smell and hearing, to detect invisible infiltrators. Armies would also fly trained hawks, eagles or vultures overhead to attack enemy familiars sent on reconnaissance. The cost difference between the countermeasures and the reconnaissance assets meant that even an army with inferior resources could considerably mitigate the amount of information gathered by a superior army.
Meanwhile, miliaries and governments, made extensive use of Animal Messenger (2nd, ritual) and Sending (3rd) (and Sending Stones (Uncommon)) for coordination between separated units and with HQ. By and large, there were no practical means to prevent the enemy from communicating nor to intercept its communications.
The Age of Emancipation saw the introduction of several new improvements that really upended the equilibrium. The development of Improved Arcane Eye (?th), which allowed the creation of a highly mobile invisible sensor a mile way with reasonable optical capabilities, rapidly overtook the use of familiars in general reconnaissance. Since it could not pick up sound, it had limited use in surveillance, although, many wizards started training themselves in lip reading. That spell was first used by the nascent Thamassan Empire but within a few decades, it had percolated throughout most kingdoms.
At about the same time, the Tasnican Mages started deploying tents and tarps with a particular magical pattern weaved into it, often concealed to the naked eye. The fabric functioned as a complete block for remote sensing (Arcane Eye), perception through solid objects (Ghostly Gaze) and any form of communication prevented by being on separate planes. The magical pattern could also be invisibly applied to walls and windows, and over time most important buildings within the Tasnica Republic were covered, on the inside, with the pattern referred to as Weather Shielding. It took centuries for the secrets of Weather Shielding to leak out to the non-Mage spellcasters, but by the start of the Great War, the Thamassan Empire, also made extensive use of it.
Communications were also greatly improved during this period of time. Wizards working for the various Banking houses developed a network of Sending Monoliths and offered, somewhat, affordable and rapid messaging services between any major cities and eventually extended coverages to most towns. Improved Sending Stones were also marketed to military and merchants, and allowed near unlimited use of Sending between linked stones. This improvement in communication greatly facilitated the operation and coordination of large armies and fleets.
On the other hand, by the end of the Age of Emancipation, it became clear that the Mages were able to intercept most, if not all, magical forms of communications including the communications between a familiar and its master. To what extent and under which conditions were not well understood. Also, Mages were also able to intercept or deny Teleportation, even between directly between permanent circles.
At that point in history, the Tasnican Republic seemed to have an overwhelming advantage over the Empire in most aspects of intelligence gathering. Some historians have theorized that the Great War could not have occurred, or would have been quite short, without the groundbreaking contribution to magic from the Banking Federation. As the network of Sending Monoliths grew, it became an integral part of the Banking Houses business and their ever deepening ties into regional partnerships and alliances. One of the pillars of the success of the Banking Houses was the absolute secrecy that they offered to their clients. The second pillar was the communication network itself, which allowed money to be sent, notionally on ledgers or physically teleported. Finally, the messaging services of the Banking Houses had become quite profitable itself and its confidentiality was central to its value.
It was the Orenzian Banking Alliance, on the Eastern Plateau that develop the arcane means to authenticate parties and cipher the content of magical communications. The Alliance successfully leveraged this advantage to convince and coerce other banking groups to join them, under favorable terms, into what would become the Acarn Banking Federation. Try as they might, the Federation was unable to keep the secrets of Cipherology from the Empire or the Republic for long.
At the dawn of the Great War, all States and many non-state actors, had the means to securely communicate across arbitrary distances and protect themselves from scrying. But that was only the beginning of the story. As the war marched on, both sides spent considerable amounts of resources in trying to break or circumvents the other’s scrying defences.