Donald Trump used the second veto of his presidency to defeat a congressional resolution that recommended that the U.S. withdraw all support for Saudi Arabia in its war against Iranian-backed Houthi rebels in Yemen.

In vetoing the measure, Trump asserted his presidential prerogatives as commander in chief to protect U.S. civilians and soldiers. 

Associated Press:

"This resolution is an unnecessary, dangerous attempt to weaken my constitutional authorities, endangering the lives of American citizens and brave service members, both today and in the future," Trump wrote in explaining his Tuesday veto.

Congress has grown uneasy with Trump's close relationship with Saudi Arabia as he tries to further isolate Iran, a regional rival.

Besides Israel, the Saudis have the strongest military force in the region.  They are engaged in a war to blunt Iranian influence in Yemen and beyond.  This is an effort well worth U.S. support.  And while there's no denying the growing toll of civilian casualties and the humanitarian catastrophe that's looming because of the conflict, the strategic importance of the Saudi effort cannot be overlooked.

As Trump points out, U.S. involvement in the conflict is very limited:

Trump said the measure was unnecessary because except for counterterrorism operations against Islamic State militants and al-Qaida in the Arabian Peninsula, the United States is not engaged in hostilities in or affecting Yemen.

He said there were no U.S. military personnel in Yemen accompanying the Saudi-led coalition fighting Iran-backed Houthis, although he acknowledged that the U.S. has provided limited support to the coalition, including intelligence sharing, logistics support, and — until recently — in-flight refueling of non-U.S. aircraft.

The president also said that the measure would harm bilateral relations and interferes with his constitutional power as commander in chief.

He said the U.S. is providing the support to protect the safety of more than 80,000 Americans who live in certain areas of the coalition countries subject to Houthi attacks from Yemen.

"Houthis, supported by Iran, have used missiles, armed drones and explosive boats to attack civilian and military targets in those coalition countries, including areas frequented by American citizens, such as the airport in Riyadh, Saudi Arabia," Trump said.  "In addition, the conflict in Yemen represents a 'cheap' and inexpensive way for Iran to cause trouble for the United States and for our ally, Saudi Arabia."

In truth, the Yemeni government the Saudis are fighting for does not present a shining example of human rights.  And the Saudis themselves do not have the same rules of engagement as the American military, leading them to bomb targets where civilians are likely to be killed. 

But while the U.S. role is limited, we supply vital logistical support to the Saudi air force and carry out important counterterrorism operations against al-Qaeda in the Arabian Peninsula and ISIS.  Overall, the strategic goal of preventing Iran from dominating the region should take precedence over congressional queasiness about the cost to Yemen's civilians.