In the lead-up to Donald Trump’s decision to unilaterally withdraw from the Iran deal, the President operated with near-impunity from Congress and the media. His nomination of Mike Pompeo, an avowed Iran hawk who worked tirelessly in Congress to undercut Obama’s diplomatic efforts and unravel the nuclear deal, met with some controversy but ultimately passed over the toothless opposition of Senate Democrats. Trump’s appointment of John Bolton to round out his “Iran war cabinet” provoked a handful of headlines but received far less media scrutiny than even Bolton’s 2006 recess appointment to a lower position in the Bush Administration. And in the immediate aftermath of Trump’s decision, it appeared he might also bully his way past Congress, the press, and Europe to begin escalating toward military conflict. But the tide may be turning against Trump and his “war cabinet.”

This week, Congress sent a message loud and clear to Trump: you do not have legal authorization to start a war with Iran. As part of the annual defense bill, a massive piece of “must-pass” legislation that provides the legal authority for the Pentagon’s operations, a group of lawmakers inserted a key provision to put the brakes on any plans by Trump, Bolton, or Pompeo to start a war. The language — drafted by Reps. Keith Ellison, Barbara Lee, Ro Khanna, Jan Schakowsky, Jim McGovern, and Walter Jones — amended the defense bill to state clearly that “the use of the Armed Forces against Iran is not authorized by this Act or any other Act.”

Perhaps surprisingly, the measure was adopted by voice vote with no pushback from even the chamber’s biggest hawks. In fact, this should not be a surprise. Lawmakers may be eager to beat their chests and talk about escalating pressure on Iran, but when they are forced to go on the record about actually supporting the war that may result, even the loudest Iran hawks generally go silent. Tough talk is cheap; starting a war isn’t.

If the Senate approves the House’s language, President Trump would likely be forced to sign into law an acknowledgement that he does not have the authority to launch a war against Iran. This is vital, as it would close a dangerous path that the administration may have been planning to take: citing the 2001 authorization for military force against Al Qaeda and the Taliban, the very one that has been used as legal cover for nearly all of America’s military adventures in the Middle East ever since.

Congress must now go further and shut down all of the backdoors to a war with Iran. John Bolton has made clear through his public statements and articles that his designs against Iran will begin with covert actions. In a “memo” on how Trump should kill the Iran deal that Bolton published late last year when he was barred from the Oval Office by John Kelly, he argued for propping up ethnic separatist groups inside of Iran and backing what he considers the Iranian “opposition,” the Mujahedin-e Khalq — which is in fact a cult-like group widely despised inside Iran for its history of terrorism and support for Iraq’s Saddam Hussein. And while Defense Secretary James Mattis may be the relative voice of reason on Trump’s National Security Council — he advised against tearing up the Iran deal — even he has favored stepping up U.S. efforts to confront Iran militarily throughout the region. Between Bolton’s desire for covert war to destabilize Iran and Mattis’ interest in stepped-up regional efforts, we could soon be facing a new Middle East conflagration.

That means that, instead of a one-time effort to send a political message and lay down a legal marker against war with Iran, lawmakers must work aggressively to rein in Trump’s war cabinet. That will take real political will, which may not exist in the current Republican-majority Congress. Nonetheless, the Senate must at a minimum pass its own “no authorization for war in Iran” language so it can be signed into law. Congress should also explore further options to counter Trump’s attempt to bully Europe into backing out of the Iran deal and allowing its companies to succumb to U.S. sanctions. Lawmakers should seriously consider trying to put legal limits on Trump’s ability to sanction U.S. partners who are simply complying with an Iran accord that the U.S. itself helped write.

If Trump does manage to force Europe to cave, the threat of escalation rises exponentially. A European failure to save the deal, followed by an Iranian decision to follow suit—exiting the deal, kicking out inspectors and ramping up the production of weapons-grade nuclear fuel—would accelerate the escalatory tit-for-tat set off by Trump’s Iran deal decision and lead to war.

This possibility was, in fact, what led Obama to go all in on a nuclear deal in the first place: a recognition that Iran’s nuclear program was advancing so quickly (and that economic sanctions were only accelerating that process) that the U.S. would soon have just two choices: allow Iran to develop a nuclear weapons capability or launch a war to stop it. At that time, we had a President with the discipline and foresight to avoid a lose-lose cycle and instead seek a diplomatic off-ramp. Today, we have a President who can’t even restrain himself from tweeting nuclear threats. Congress may not be able to take away the President’s iPhone, but they certainly can do far more to take away Trump’s power to start a war.