As violence rises, nations seek American defense pacts. Some Americans are wary.

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

“From a US perspective,” he said, “while there is a great deal of recognition in Washington that the unipolar moment is over, there is still a reliance on the US security umbrella to say, ‘We know how to ensure peace and stability.’ There is an assumption that the American security umbrella is the solution.”

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

Some agreements, such as new with Bahrain, represent a step down, requiring only that countries consult each other in case of hostilities. Israel is not one of them 52 allies of the United States, but some Israeli officials debated whether to insist on a formal pact.

After the attacks of September 11, 2001, the United States and its partners became more concerned about non-state adversaries like Al Qaeda and 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 improve their ties with the United States.

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

Safety umbrellas can sometimes deter opponents from attacking. Russian President Vladimir V. Putin has refrained from attacking any NATO nation even though those countries support the Ukrainian military. But the pacts can also appear tenuous: Chinese navy and coast guard ships have been acting aggressively toward ships from US-allied nations — even ramming two Philippine warships on Sunday.

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