Kentucky Sen. Rand Paul isn’t mincing words — he calls the military actions taken against the terrorist group known as ISIS “illegal” and says it must either stop or President Barack Obama needs to come to Congress to get authorization to continue it.

The War Powers Act allows the President to act militarily for 60 days before seeking congressional authorization, after which he has 30 days to seek authorization or end the action. That 90-day window has now passed. As Sen. Paul put it:

I believe the president must come to Congress to begin a war. I also believe the War Powers Act is misunderstood; President Obama acted without true constitutional authority even before the 90 days expired, since we were not under attack at that time.

But in either case, this war is now illegal. It must be declared and made valid, or it must be ended.

Congress has a duty to act, one way or the other.

Paul did not reserve all his criticism for the president; he also had choice words for Secretary of State John Kerry. Kerry “became famous as an anti-war liberal decades ago,” he wrote. “That same man is now probably the most visible liberal proponent of unlimited war-making powers, as a member of this administration.”

But Paul’s criticisms were not limited to Democrats. In a veiled swipe at Arizona Sen. John McCain, with whom Paul has had many disagreements, Paul writes, “Prominent Republicans from the interventionist wing of the party parrot and applaud Kerry’s position. If ever there was too much bipartisanship, it would be the bipartisan acceptance of unlimited presidential war-making power.”

Paul is not saying military action against ISIS is unjustified, he states the exact opposite, in fact. But it’s the process through which it is being conducted that troubles him. He sees it as a separation of powers issues and encourages his fellow senators and representatives, particularly conservatives, to reclaim the power to make war, explicitly given them by the Constitution. He concludes his piece by writing:

It’s time for conservatives to say enough is enough. Obama’s commandeering of Congress’s powers—from making war, to remaking our health-care system—has to stop. There needs to be an across-the-board, consistent defense of the constitutional separation of powers. Nothing less will win the day. That should include this current battle in the Middle East. Taking military action against ISIS is justified. The president acting without Congress is not.