Why did the global recession happen?

since the united states of america is the foremost country which affects the world's economy, what it does in all its affairs with the world, affects the world in general. we all know that wars are very costly. now, the us is very much various wars. i understand that the us of a is not necessarily a warlike country. it just so happens that it has taken into its hands the respon sibility of policeing the world over. as it does this, the us of a incurs so much expenses. whether they will deny this fact or not, the wars that the us of a is indulged in has played greatly on the downtrend of the us of a economy.