While our Constitution clarifies that only Congress has the power to declare war, it's a very different thing to suggest that the Bible justifies war in the twenty-first century. It's not as clear as some well-meaning Christians may think, and the consequences of confusion over this could be horrendously irreversible if there were to be a nuclear conflict.
While some might point to Israel's battles in the Old Testament as precedent for God using one nation to judge another, there is no direct parallel today. God's directives to Israel to obliterate enemies were given in the context of his plan for Israel, revealed in Genesis 12 with the Abrahamic Covenant. What God did with Israel was not done as an example for other nations to follow; His directives had a very focused purpose for a very specific time. The United States is not Israel. We cannot apply the directives God gave to Israel to the United States. Doing so is not only poor biblical exegesis, but it could also get tens of thousands of people – or more – killed.