Isis is conquering parts of Kurdistan with the help of heavy US weapons captured from the disintegrating Iraqi army.

Who Are the Yazidis And Why Is Isis Hunting Them?

Obama announced he will launch air strikes to prevent genocide.

That is a valid reason for war, and supporting Iraq in defense makes it legal as far as I know.

I worry, however, that this US intervention will backfire just like the last one. In the long term, it would make more sense to give heavy weapons to the Kurds. They probably can be relied on not to lose those weapons to Isis.