Qatar hosts the largest American military base in the Middle East, has bought billions of dollars worth of defense systems from the United States and recently gifted a luxury Boeing jet to President Trump.

Yet on Tuesday, none of that stopped Israel, a key U.S. ally, from launching a brazen military attack on Qatari soil. It was an attempt to assassinate senior Hamas officials who had gathered to discuss a cease-fire proposal to pause the war in Gaza — a deal that was backed by Mr. Trump.

“Qatar being unable to protect its own citizens with literally the U.S. Central Command on its territory has prompted locals to question the value of the American partnership,” said Kristin Diwan, a senior resident scholar at the Arab Gulf States Institute in Washington, a research group. “It’s a real problem for Gulf leaders. And it should worry the United States as well.”

The Israeli attack sent shock waves through Gulf capitals that have been courted by Israel as potential allies in recent years and have long regarded the U.S. as their main security guarantor.

The strike hit a residential neighborhood in the Qatari capital of Doha, sending black smoke into the sky and killing a member of Qatar’s internal security forces, Bader Saad al-Humaidi al-Dosari, according to Qatari officials. He became the first Gulf Arab to be killed by Israel in decades.