ON Thursday, the US House of Representatives passed a resolution affirming its authority under the War Powers Resolution Act, and reminding the president, the American people and the world that when it comes to the use of military force, Congress must be taken into account.

President Donald Trump replied by retweeting John Bolton, his former national security advisor, who posted that the War Powers Act was “unconstitutional”, effectively dismissing Congressional efforts to rein in the president.

This round of legal Twitter diplomacy came days after the president tweeted that “legal notice [to Congress] is not required, but it is given nevertheless” and that “should Iran strike any US person or target, the United States will quickly and fully strike back, and perhaps in a disproportionate manner”.

President Trump’s overt attacks on Congressional authority and disregard for the law are not normal for a US president, but legal constraints over the president’s authority to use military force have been eroding for several years.

In 1973, Congress passed the War Powers Act, requiring the president to report to Congress and acquire Congressional approval for continued overseas troop deployments. But Congressional authority over the use of military force has continued to diminish, especially since the 9/11 attacks.

Now, Congress is pushing back – a resolution similar to that passed by the House is likely to be voted on by the Senate soon. Given the intense frustration of some senators over the president’s handling of the current conflict, this resolution may pass, though the president is still likely to veto.

The bigger problem, of which this is one part, is the expansion of executive authority, and it predates this presidency. Many argue executive power is greatest on matters of foreign policy, and that in this domain the US president is unconstrained. As partisanship has increased and the demands on Congress have made it harder for its members to cultivate foreign policy expertise, the power of the president has grown.

So the fact that Congress is taking active steps to check the president’s power not only on the question of impeachment, but also on foreign policy, is significant, even if the law is not likely to restrain president Trump. But Congress is also playing a long game. And its growing determination to assert its authority matters not only for this presidency, but also for the future.

In the short-term, other constraints matter more. Even if president Trump rejects legal restraints – and shuns diplomatic pressure that might have mattered to other presidents – he is not impervious to political restraint.

Already there are signs that Trump listens to his key Republican allies in Congress. When Senator Lindsey Graham pushed back against the president’s threat to attack cultural heritage sites in Iran, and the Pentagon failed to shore up the president’s threat, the president changed his position. The threats also provoked an overwhelming response in Europe, and across the Middle East, but the president has shown that he is willing to disregard international admonishment. Not so for domestic allies.

Nor is this the first time that senator Graham’s views have mattered. When the president announced his decision to pull troops from north-east Syria, effectively abandoning the Kurds, a key partner to the US in its fight against ISIS, senator Graham and Senator Mitch McConnell pushed back and it mattered.

Public opinion also matters to the president, especially Republican public opinion, and this is likely to affect the president’s next steps. Recent polls suggest that Americans feel less safe as a result of the killing of general Qasem Soleimani by a considerable margin of 55% to 24%.

But the same survey also shows that a majority of Americans support the decision to kill Soleimani. Already Vice President Mike Pence has spoken at a rally in Ohio designed to bolster support for this presidential action.So what might change public opinion and will this alter the president’s future steps?

Americans are very reluctant to support further deployments of troops overseas. President Trump knows this and he has consistently stated that he seeks to pull US troops out of the Middle East. In fact, the opposite has happened. And once again, the decision to kill general Soleimani is pulling the US further into the Middle East and putting into stark light the inconsistency of Trump’s foreign policy ambitions. In recent days, the United States has committed to sending thousands of troops to the region.

Continued US troop deployments may not play well with Trump’s base, though they currently support his Iran policy. But absent an obvious justification, this could change, so it is not surprising that this has become a rallying point for vice president Pence. An even greater political risk, and one that will matter to the president, is the risk to US military personnel and civilians overseas. Public support for the killing of Soleimani and increased US presence in the region is bound to dissipate if Americans are killed.But it isn’t clear how the president will respond, and already the death of one American in the militia attack on December 27 seems to have been a critical factor in Trump’s decision to authorize the assassination of general Soleimani. So far, Iran’s response appears to be calibrated to avoid this kind of public backlash, and keep the door open to some sort of managed resolution of the current crisis.

But the considerable risk of miscalculation and mistakes has already been born out. A week after the killing of Soleimani, the world is tuning in to the tragic news that a civilian airliner departing Iran for Ukraine appears to have been accidentally shot down by an Iranian missile, resulting in the deaths of all 176 people on board, including 82 Iranians and 63 Canadians.

Whether the US president is constrained is a critical question for Americans, for US democracy and for the rest of the world. Trump is different, but he is not unconstrained. Steps taken by Congress along with diplomatic and political pressure mean he is still operating within boundaries. But it is clear that these boundaries are being redefined.

This article was originally published by Chatham House