In Medieval movies and video games a country's military is often all at the same place, a big army and one huge battle but why? After 1900 battles are often fought at different places and spread out.

That was mostly how wars were fought back then. Excepting the small skirmishes involving just a few dozen or hundred men (depending on the place/time period) and sacking of small villages and towns, most battles were fought in larger battles in which the majority of the soldiers of each side participated.

This kind of stems from (in the West, at least) ancient Greece, where warring city states only had the population to support a small army; once the two armies fought a battle, the winner of that battle would usually be considered the victor in the war, as the other side's population would not be large enough to replace the losses the army sustained. This sort of continued into the Middle Ages, when Medieval counts, princes, dukes, and kings would levy their people, marshal the troops together, and march off to defeat the enemy in one decisive battle (or several). This made organization and logistics [somewhat] easier; the ruler had immediate command over the troops that he marched with. Even if he could manage to army, feed, and levy an army of 100,000 men, and then have them fight in a more "modern" style (fighting on all fronts), he would not be able to give orders to these men simultaneously; some would fall under the command of his subordinate officers. Some monarchs would presumably not like that and would prefer to have their men under their command in their entirety. Not to mention that it would be kind of stupid for anyone to attempt to cover a vast hundred-mile front (or more) with just a few thousand men. It makes more sense to gather every soldier you can and to then march off in search of the enemy.

As technology developed at the beginning of the modern age, we see an improvement in general knowledge of maladies, disease, and treatment of people for sicknesses and wounds. This continued throughout the nineteenth century (which still saw pitched battle-style warfare right until the last decades). By 1914 the major nations of the world had what the kings and princes of centuries (and millennia) before lacked:

  1. A large population (Germany had close to 65 million in 1914, France had roughly 40 million, and Russia had at least over 180 million

  2. Widely-spread and developed transportation networks, most importantly of which were railroads.

  3. Enormous economies with thousands of factories, all of which could be retooled to produce weapons of war.

An increased population meant there were more men to be conscripted into the army, leading to millions of men fighting on the frontline simultaneously. Expansive industries producing thousands of rifles a day and hundreds of artillery pieces meant that knock-out blows (large groups of men and guns being captured) were still devastating, but not war-ending; and vast railroad networks meant that these new conscripts and weapons could be rapidly deployed to where the armies were mobilizing (on the front line).

/r/AskHistorians Thread