Why does the US army fight?

Nations go to war; armies are merely one of the means by which wars are fought. So the question should be, why do nations go to war? The short answer is, when two nations (or two allied groups of nations) have a disagreement which they are unable or unwilling to resolve by peaceful means, they will often resort to violence. This is often a very unreasonable and cruel thing to do, which causes death, destruction, and suffering for large numbers of people, but then national governments are often unreasonable, and the populations that they govern, who may be responsible for electing them, are also often unreasonable. The human race has its flaws.