Well, this whole Iraq war is a very complicated issue.
Actually, the first one was complicated too: but in large part it was because Kuwait was getting Oil from their "Northern Fields" which were really dug underground many miles across into Iraq. So, Iraq was pissed that Kuwait was stealing Iraqi oil - and the US wanted to ensure Kuwait could access that oil to keep the price low.
The second Iraqi war was, in large part, because Iraq had started demanding that its oil was paid for in Euros rather than dollars. This is a trend in the middle east (and in fact other countries) that international trades are starting to happen in Euros and not (as before) dollars. This means that foreign countries want more Euros and fewer Dollars. Hence, dollars are losing value rapidly. In fact, one thing that keeps the US economy propped up is that something like three quarters of all US$ are actually held overseas - well, if countries stop "stockpiling" dollars and switch to holding Euros, it will make the Dollar lose value like you could not believe. In theory, a dollar could be worth a quarter of what it is worth today. Now, this would completely devastate the US economy. So ... a few wars are a "reminder" to a number of countries that if the US economy is threatened there will be retaliations.