capri0mni | (Reply)

From:

One of the traditional purposes of war is conquest - the acquisition of one's neighbour's territory in order to gain more fertile land, or a more secure border with a third country, or access to a useful sea or river port, or the taxes from lots of wealthy cities, or whatever.
Somewhere around the 1920s it was suddenly decided that this was no longer permissible and that the existing borders should be set in stone and defended by international treaty (although that didn't work out too well in the immediate future) - before that, it was taken for granted that maps shifted around as countries fought one another.

Invasions by people like the Mongols (Huns, Visigoths, Cossacks, whatever) are more akin to animal movements where one population expands its territory at the expense of another; you simply have two peoples competing for the same resources, one settled and the other highly mobile, and neither really recognising the other as an equally valid way of being human...

Another traditional purpose of war is 'the continuation of diplomacy by other means' (i.e. forcing another country to sign up to a treaty or avenging a perceived insult -- when discussions between governments fail, you declare war).

If you have an empire (like the Romans or the Persians or the Zulus) then you have an army to maintain that empire and stop other people nicking bits of it around the edges, or bits of it deciding to revolt and refuse allegiance to you.

The religious wars in Europe were supposedly about saving people's souls by forcing them to worship the right way, although they were also about which rulers were going to allow the Pope to tell them what to do...

The actual killing is not (usually) the aim. The aim is winning, and you win by making it impossible for the enemy to continue fighting back. Unfortunately nobody ever starts a war unless they think they're going to win it - they're often wrong to the tune of large numbers of lives.