Why iraq's people kill soldiers

U.S. and U.K. release iraq's people from suppression.
But iraq's people kill soldiers of U.S. and U.K.
Why?
 
Please teach...

In a sense that's right. I'd better say that why there are
a lot of wars in religious countries. It's hard to believe
us, who live in a non-religious, namely very secular nation.
Could you give some hint?