Yesterday, the US and UK launched air and missile strikes on Houthi forces in Yemen, in response to the latter's repeated attacks on shipping in the Red Sea. This action raises a variety of moral and policy issues. It also raises the question of whether President Biden has the authority to launch these strikes without congressional authorization (which, so far, he hasn't gotten). Several members of Congress - mainly on the far right and far left - have already claimed Biden acted unconstitutionally.
It is true that the Constitution gives Congess, not the president, the power to declare war. The president cannot initiate any large-scale military action on his own. But critics of Biden's action overlook the fact that the US strikes are not initiating a war, but responding to attack. For weeks, the Houthis have been launching indiscriminate attacks on shipping in the Red Sea, one of the world's most important waterways.
As legal scholar Michael Ramsey shows in his excellent article, "The President's Power to Respond to Attacks," the president does not need advance congressional authorization to respond to attacks on US troops, territory, or American ships on the high seas. In such cases, it is not the US that has initiated the conflict, but the enemy. Ramsey also makes clear that defensive responses may include tactically offensive actions, such as - in this case - targeting the bases and other facilities the Houthis used to launch their attacks on shipping.
There is, admittedly, a potential complication here, in that it's not entirely clear the Houthis were targeting US ships. At least initially, they claimed they were only attacking ships with connections to Israel. However, they have hit (or tried to hit) ships from a variety of nations. That strongly suggests that either they don't really care whether a ship has connections to Israel, or their definition of what qualifies as a connection is so broad that almost any ship in the Red Sea potentially qualifies.
Moreover, earlier this week a Houthi spokesman actually admitted that one of their recent attacks "targeted an American ship that was providing support to the Zionist entity." Needless to say, the US ship in question wasn't actually "providing support" to Israel in any but a highly tangential sense that could apply to almost any US ship. And even if the ship was in fact somehow helping Israel, the Houthi action still qualifies as an attack on the US.
In addition to targeting at least some US ships, the Houthis have also been attacking ships belonging to US NATO allies (such as Norway). Under Article 5 of the NATO treaty, the US has a legal obligation to help defend other members of the alliance if the latter are attacked. That provides further legal justification for Biden's strikes. By ratifying the North Atlantic Treaty, Congress has in effect preauthorized military action when necessary to carry out US obligations under Article 5.
In sum, there is likely solid legal justification for US strikes against the Houthis - at least so far. I have been highly critical of previous military actions that lacked proper constitutional authorization, such as Barack Obama's 2011 intervention in Libya (which lacked congressional authorization, and was not a response to attack). But this case is different.
That doesn't mean Biden has completely unlimited discretion to use as much force as he wants in this situation. If he tries to expand the conflict beyond anything that could reasonably be considered defense against attack (e.g. - a large-scale ground invasion of Yemen), that would likely require congressional authorization. That is especially true if the president wants to strike the Houthis' sponsor, Iran, as well as the Houthis themselves.
In addition, if this military intervention goes on for more than three months, it might run afoul of the 1973 War Powers Act, which requires the president to get congressional authorization for any use of military forces in "hostilities" abroad within 90 days. Some argue that the War Powers Act is unconstitutional. But the Biden Administration hasn't taken that position, and I think they are right not to. And this conflict pretty obviously qualifies as involving US troops in "hostilities."
For these reasons, the White House may be well-advised to try to get Congress to pass an authorization for the use of military force (AUMF) for this situation. It should not be hard to do. While congressional Republicans are at odds with Biden on many issues, when it comes to countering Iran and its proxies (including the Houthis), most of them are actually more hawkish than the White House, and have been hammering Biden for not doing enough. An AUMF might also have deterrent value by sending a signal of unity and resolve to the enemy.
Be that as it may, it seems likely that - so far - US military actions against the Houthis are legally justified. Unlike some previous military interventions, they don't violate the constitutional separation of powers. Whether that remains true depends in part on the scope and duration of any further military action.
UPDATE: Michael Ramsey comments on the legal issue and this post here. He in large part agrees with my analysis, but offers some caveats. The most notable is he doesn't think Article 5 of the NATO treaty allows the president to use military force to defend an alliance member from attack without additional congressional authorization. To my mind, ratification of the treaty qualifies as such authorization in the event of attack on treaty member, just as Congress can enact advance authorizations for the use of military force (AUMFs) for other situations - ones triggered by specific events. Examples include the 1991 and 2002 AUMFs giving advance authorization for the Gulf War and the Iraq War, respectively.
