Romans 13:3,4

These verses are talking about government:

"For rulers [governments] are not a cause of fear for good behavior, but for evil. Do you want to have no fear of authority? Do what is good and you will have praise from the same; for it is a minister of God to you for good."

But if you do what is evil, be afraid; for it [government] does not bear the sword for nothing; for it is a minister of God, an avenger who brings wrath on the one who practices evil." - NASB

In the first century the sword was the chief weapon of war, and to "bear the sword" refers to going to war.

The Bible is clear that as individuals we are not to seek revenge against those who do us wrong. However, the actions of governments, all of which are given to us by God (Romans 13:1), are governed by different rules. Governments are given the authority, and responsibility, to oppose evil. That's what America did in World War II when we entered the war against Hitler and Germany. That is what America is now doing in Iraq.