On the other hand, biological agents have made war cheaper than ever too. A canister of Ebola in a crop-duster can wipe out a city. Governments have taken extraordinary steps to regulate biolabs and register/certify researchers working in potentially dangerous fields. Preemptive strives to take out an enemy's biological capabilities might be considered a necessity of war.
The most significant war of the century was the Mars War. As space became a greater and greater part of the economy, the rights of nations and corporations to exploit it became a more contended issue. The abstract 20th century treaties of equal rights to space and bans against weapons in space were undermined by the economic realities of later times. Space was too profitable for such altruism. Although its origin will naturally be argued forever, without a doubt the Mars War had its roots a conflict over rights to mine for water. The Mars China nation had been the dominant power on the red planet for more than 40 years. It sought to maintain its position by controlling the planet's economy by controlling access to the stores of water frozen in the polar caps. While it made sense for many years for smaller colonies (and even cities) to tap into China's water pipeline, the burden of China's prices and occasional water embargoes were too much to bear. The pipelines were sabotaged several times in the early 2070's, while the Republic of Tharsis tried to build their own pipeline when some of their own major water mines tapped out.
Other tension came from the territory disputes in the Belt. Despite the vast amount of space and number of rocks in the Belt, the settlements and corporations still rubbed against each other and struggled for control of the largest asteroids. In 2074, the Kilimanjaro Mineral Trust corporate freighter, Fenton Beach, was hijacked by a group of former employees. After stopping at a small rock to be loaded with explosives, the freighter was piloted back to the KMT Belt Headquarters on the major rock Cillarne and detonated at a loading dock. The total destruction of the base inspired an arms escalation among the Belters.
The conflict came to a head in 2075 when a Russian ship dropped a fusion bomb on Mars China from orbit, flattening the city of Hsuhsu. So began the first interplanetary war. The conflagration ebbed and flowed until the Beijing Treaty of 2086. At one point, it seemed that the Alpha Cen colonies might even be dragged in, but the long distance communications prevented that.
It is possible (or even likely) that the PCs may have served in a national
or corporate military force, depending on their place of origin.
With many exceptions (especially the Mars War), they would probably have
seen most of their action through a computer link to a battle site far
away.