The late 18th and early 19th century is really when the last of European monarches led armies to the field. For example, George II was the last reigning king to lead an army for the British, while Fredrick Wilhelm III nominally operated as the commander of the Prussian Army during the Battle of Jena–Auerstedt in the Napoleonic Wars, although in practice he allowed subordinates to do most of the actual leading. Those Napoleonic wars essentially represent the last gasp of monarchs, and for the most part, even on Napoleon's part, subordinates acting in the name of the monarch were the order of the day. Why this happened is not too difficult to figure out; as Europe was consolidated into a smaller number of large and increasingly centralized polities over the course of the 18th and 19th centuries, the need (and, some might say, the ability) for monarchs to lead their armies in the field disappeared. Feudalism meant that not only was a king needed to rally the various dispate liege levies, but also he by necessity would have wanted to be the one in control of those levies. With the rise of professional armies of increasing size and the appointment of professional officers to the command of said armies, armies whose loyalty was owed directly to the monarch, there was no longer any need to lead those armies directly. Indeed, the only value that could be derived therein was a propaganda one, and in that capacity subordinates played a crucial role. A more modern example is WW2 Russia, where Stalin was the ultimate commander of the Soviet forces. Obviously he wasn't the one planning offensives, but he still took the credit for the victories his armies won. Conveniently, defeats were always the fault of the man on the ground.