As Violence Surges, Nations Seek U.S. Defense Pacts. Some Americans Are Wary.

“U.S. allies and partners are concerned about U.S. overstretch,” said Stephen Wertheim, a historian of American foreign policy and senior fellow at the Carnegie Endowment for International Peace. “But paradoxically, if they have put enough chips to date in the U.S. basket” of military ties, he added, “they want to seek closer relationships and more formal commitments from the United States.”

“From the perspective of the U.S.,” he said, “although there is great recognition in Washington that the unipolar moment is over, there’s still a reliance on U.S. security umbrellas along the lines of, ‘We know how to provide for peace and stability.’ There’s a presumption that the U.S. security umbrella is the solution.”

Defense agreements between the United States and other nations come in all shapes and sizes. The strongest, which usually need approval by two-thirds of the Senate, guarantee mutual defense if one country is attacked. Article 5 of the NATO charter is a prominent example.

Some agreements, such as the new one with Bahrain, are a step down, requiring only that the countries consult with each other in the event of hostilities. Israel is not one of the 52 treaty allies of the United States, but some Israeli officials have discussed whether to push for a formal pact.

After the attacks of Sept. 11, 2001, the United States and its partners became more concerned about nonstate adversaries like Al Qaeda, and they focused their energies on the so-called global war on terror. But in recent years, as Russia and China have acted with greater military aggression, and as Iran and North Korea have advanced their nuclear and missile programs, many countries have sought to upgrade their ties with the United States.

The strategies in this era of so-called great-power competition hark back to the alliance and bloc building that took place during the Cold War.

Security umbrellas can sometimes deter adversaries from attacking. President Vladimir V. Putin of Russia has refrained from strikes on any NATO nation even though those countries support Ukraine’s military. But the pacts can also appear flimsy: China’s naval and coast guard vessels act aggressively toward the ships of countries that are U.S. treaty allies — even ramming two Philippine military vessels on Sunday.