I'll show you a casus belli
The U.S. claims that the Iranian government is sending explosive devices across the border to aid anti-American insurgents. War-mongering idiots are claiming that this provides the U.S. with a "casus belli" to wage war on Iran. Putting aside the fact that the only evidence for the accusation is the assurances of U.S. officials, it should be kept in mind that the U.S. has been conducting reconnaissance missions inside Iran for some time now, which itself could be seen as an act of war and thus a casus belli for Iran to attack the United States.
But we all know that the U.S. plays by its own rules. So how about we can the Just War Theory bullshit, then? If the right-wingers want to agitate for unprovoked aggression, they should at least have the balls to admit to it.