The deployment of an advanced United States anti-missile system to Israel, along with 100 troops to operate it, marks a significant escalation in US entanglement with a widening Israeli war that Washington has already heavily subsidised.

But the deployment – in anticipation of an Iranian response to an expected Israeli attack on Iran – also raises questions about the legality of US involvement at a time when the administration of US President Joe Biden is facing growing backlash over its unwavering support for Israel. It also comes as US officials are seeking to project authority and threatening to at last enforce US law prohibiting military aid to countries that block humanitarian aid, as Israel has regularly done in Gaza.

Two recent developments — the Sunday announcement that the US would deploy troops to Israel and a letter sent by US officials the same day calling on Israel to improve the humanitarian situation in Gaza or face unspecified consequences — underscore the inconsistent approach of an administration that has effectively done little of substance to rein in Israel’s ever-widening war.

At a press briefing on Tuesday, State Department spokesman Matthew Miller declined to say what the consequences of Israel failing to comply with US requests would be, or how this differs from an earlier, unfulfilled threat by the Biden administration to withhold military aid to Israel.

“I’m not gonna speak to that today,” Miller told reporters when pressed for details of how the US would respond to Israel’s failure to comply.

Empty threats

In the private letter, which was leaked on Tuesday, US Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin called on Israeli Defence Minister Yoav Gallant and Minister of Strategic Affairs Ron Dermer to implement a series of “concrete measures”, with a 30-day deadline, to reverse the deteriorating humanitarian situation in Gaza. The US briefly paused the delivery of thousands of bombs to Israel earlier this year as Israeli officials planned to expand their operations in southern Gaza, but it quickly resumed and continued supplying Israel with weapons even as it escalated its assault in Gaza and later in Lebanon.

“A letter jointly signed by both the secretary of state and secretary of defence indicates a heightened level of concern, and the not-so-subtle threat here, whether the administration carries through with it or not, is that they will actually impose consequences under these various legal and policy standards,” Brian Finucane, a former legal adviser to the US State Department and senior adviser with the US programme at the International Crisis Group, told Al Jazeera.

Whether the administration would carry through with it remained very much in question.

“It’s important to note that there were legal standards during the entire course of this conflict, and the Biden administration has just not enforced them. It may be the situation is so dire in northern Gaza that the political calculations have changed, and that they may actually finally decide to implement US law. But it’s really long past the point at which they should have done so,” Finucane said.

Finucane also noted that the 30-day deadline would expire after the US presidential election next month. “So they may feel that whatever political constraints the administration may have felt it was operating under, they may feel less constrained by,” he said.

Miller, the State Department spokesman, told reporters on Tuesday that the election was “not a factor at all” — but Annelle Sheline, a former State Department official who resigned earlier this year in protest of the administration’s Israel policy, disagrees.

“I interpret it as being intended to try to win over Uncommitted [National Movement] voters and others in swing states who have made clear that they are opposed to this administration’s unconditional support for Israel,” Sheline told Al Jazeera. “I do not expect to see consequences.”

Deeper entanglement

Whether the US would carry through with its threats, the deployment of troops to Israel sent a much more concrete message of ongoing US support no matter how dire the humanitarian situation.

The US-made Terminal High Altitude Area Defense system, or THAAD, an advanced missile defence system that uses a combination of radar and interceptors to thwart short, medium and intermediate-range ballistic missiles, adds to Israel’s already extraordinary anti-missile defences as it weighs its response to an Iranian missile attack earlier this month. Biden said its deployment is meant “to defend Israel”.

The announcement of the deployment came just as Iranian officials warned that the US was putting the lives of its troops “at risk by deploying them to operate US missile systems in Israel”.

“While we have made tremendous efforts in recent days to contain an all-out war in our region, I say it clearly that we have no red lines in defending our people and interests,” Iran’s Foreign Minister Abbas Aragchi wrote in a statement on Sunday.

In practice, the deployment further drives the US into war at a time when US officials continue to pay lip service to diplomacy.

“Rather than force de-escalation or act to rein in Israeli officials, President Biden is redoubling efforts to reassure Israeli leaders that he is in lockstep with them as they deliberately barrel towards regional war and escalate a genocidal campaign against Palestinians,” Brad Parker, a lawyer and associate director of policy at the Center for Constitutional Rights, told Al Jazeera.

Parker and other lawyers argue that the Biden administration is relying on narrow and stretched legal arguments in an attempt to justify a seemingly unilateral move under US law. The US is also already implicated under international humanitarian law for the support it has given Israel as it violated the laws of war.

“So far, the Biden administration has tried to characterise the fortification of existing deployments and authorisation of new deployments as fragmented or individual incidents. However, what emerges is a comprehensive and robust introduction of US forces into situations where involvement in hostilities is imminent without any congressional authorisation as required by the law,” Parker said.

“All Americans should be seething that a lame duck president is clinging to narrow legal interpretations that cut against the clear intent of existing US law to justify the massive deployment of US forces into a regional conflagration that was in part created as a result of his own destructive, genocide-supporting policies.”

No congressional approval  

Experts say that deploying US troops equipped for combat anywhere in the world and without congressional approval, as Biden is doing now, could trigger US laws that require reports to congressional committees. Should the deployed troops engage in certain actions – in this case, using the THAAD missiles – it would start a 60-day clock for their removal, or for Congress to sign off on further engagement.

“This does, in my view, constitute the introduction of US armed forces ‘into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances’,” Oona Hathaway, director of the Center for Global Legal Challenges at Yale Law School, told Al Jazeera, citing the federal law regulating the president’s authority to commit the US to an armed conflict. “And therefore [it] ought to be authorised by Congress”.

But the US has been quiet about the legal implications.

“The Biden administration has gone out of its way to avoid acknowledging the application of this law,” said Finucane. “Because one, this law imposes constraints, the 60-day limit on hostilities; and two, if the Biden administration acknowledges that this law is in place and the constraints apply, it doesn’t have attractive options. It can either stop the activity or go to the US Congress for a war authorisation. And it doesn’t want to do either of those.”

This wouldn’t be the first time the administration has downplayed its legal obligations as it entangles the US in conflicts abroad. The US has, for instance, been fighting Yemen’s Houthi rebels since October 7 without congressional approval.

The Biden administration has justified those military operations as “self-defence” — something it may try to do again. The US Defense Department did not immediately respond to a request for comment.

“Thus far, Congress has not required the administration to explain how exactly Iran firing on Israel undermines US security,” said Sheline, the former State Department official. “It’s possible that Biden anticipates that Iran will attack and Congress will then be eager to declare war.”