America has many allies. Maybe too many.

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The United States is picking up new allies everywhere. Kenya just became its 19th so-called “major non-NATO ally,” a status that gives the country privileges in military procurement and cooperation but stops short of explicit security guarantees. Saudi Arabia, meanwhile, has reason to hope for the full monty, as it closes in on a mutual-defense pact with Washington.

Those partnerships follow Sweden joining NATO as its 32nd member this year, and Finland as the 31st last year. And in the Indo-Pacific the U.S. is constructing new geometries among allies such as Japan, South Korea, Australia and the Philippines — into “trilaterals,” “quads” and other configurations, some with names (AUKUS, I2U2) that sound like characters in Star Wars sequels.

This proliferation of American alliances is not tangential, but central to the foreign policy of President Joe Biden. “America’s alliances are our greatest asset,” he intoned upon taking office. By that accounting, the U.S. balance sheet is looking stronger than ever.

Biden, of course, is also reacting to his neo-isolationist and undiplomatic (in the dual sense of lacking foreign-policy finesse and tact) predecessor in the White House. Donald Trump snubbed allies whom he considered to be freeloaders on American military prowess and spending, and even cast doubt on the mutual-defense guarantee that underpins NATO.

By contrast, Biden wants to reassure America’s friends, while deterring foes from testing Washington’s security commitments. (Notably, Ukraine, which is not an ally, was invaded; that explains why Kyiv is now pushing desperately for any kind of security pact with the U.S.)

But there’s a countervailing view. If the U.S. commits to defending so many friends against so many foes, is it not overextending itself and thereby undermining its credibility? This risk grows when Washington’s “mutual” defense pacts are in fact one-sided. Would Iceland, a NATO member that has no army, really come to the aid of the U.S. in a war against China? Would Germany, which spent several decades demilitarizing? Would Saudi Arabia actually help fight Iran, or rather leave it to the Marines?

This multi-front exposure, via the various alliances, could invite an even worse scenario. In protecting Estonia or Lithuania, the U.S. potentially confronts Russia; in support of Saudi Arabia or Israel, it could face Iran; standing with the Philippines, it may need to oppose China; by defending South Korea, it might have to fight the North, and so on. These adversaries increasingly behave as an anti-Western and anti-American “axis.” What’s to keep them from coordinating, on the plausible theory that the U.S. couldn’t defend all of its allies simultaneously? In that way, conflicts that are now discrete — in the Taiwan Strait or the Persian Gulf, say — could one day become theaters in a single world war.

To weigh the pros and cons of alliances with more precision, I chatted with Christopher Preble at the Stimson Center in Washington. He’s not an isolationist, but a habitual contrarian who enjoys untangling the “fuzzy” thinking that often passes for analysis in Washington. Here are some guidelines we came up with.

Alliances make most sense when — and only as long as — the U.S. and one or several allies share the same strategic interest; the more specific, the better. The best example is NATO. During the Cold War it united countries that felt threatened by the Soviet Union. After the Cold War, that unifying interest temporarily waned, causing an identity crisis for the alliance, but then came back with the aggression of Russia under Vladimir Putin. This menace is the reason why Sweden and Finland decided to join, and also why their membership now makes the whole alliance stronger.

But even NATO has design flaws. Like the European Union (but unlike the United Nations, say), it has no mechanism to expel members who become unreliable over time. Early in the Cold War, Washington worried that Italy might turn communist and become a Trojan horse. These days there are concerns about Turkey and Hungary, which might, in a pinch, side with Putin over their allies.

America’s quasi-allies — those without explicit security guarantees — present another problem: moral hazard. Assuming that Washington has their back, U.S. partners may engage in the diplomatic or geopolitical equivalent of reckless driving. What if Pakistan gets too pushy with India? And what about Taiwan, which the U.S. doesn’t even recognize as a state but typically includes in the category of major non-NATO ally? Israel, also in that group, currently presents dilemmas in a class of its own.

These partnerships are inherently ambiguous. Major non-NATO allies don’t have mutual-defense pacts with the U.S., and yet they and the U.S. often create the impression that they do, as when Biden swears that his defense of Israel is “ironclad.”

This again raises the question of U.S. credibility. If the U.S. doesn’t come to the aid of Israel, Taiwan or Kenya, will its adversaries and its treaty allies — those with security guarantees — infer that Washington has gone wobbly? Preble thinks that “credibility doesn’t travel that way,” because the only thing that matters, or should matter, is what U.S. interest is at stake in one place at one time. But “precisely because we treat all our alliances as tests of American credibility, we open the door to somebody testing it.”

Another worrying tendency is for the Biden administration to mix ulterior rationales into its quasi-alliances. It wants a pact with the Saudis, for example, not only to deter Iran but also to keep the Chinese at bay, and to pressure the Israelis to start working toward a Palestinian state in return for détente with the Saudis. Biden was keen on the deal with Kenya in part because he recently lost influence in other countries in Africa — Niger, following a coup, is kicking out the remaining American soldiers stationed there by September — and the Russians are moving in wherever the U.S. withdraws.

But there is nothing to keep America’s allies, quasi or full, from flirting with Beijing and Moscow all the same. If exclusivity is Washington’s objective in a relationship, why not make it a clause in the agreement? If that’s too off-putting to the partner nation, why not settle for a trade deal, and leave out military support for the time being?

Alliances, it turns out, are like all commitments. It’s not how many you make, but how well you choose them, how conscientiously you withdraw from those that are no longer tenable, and how devotedly you honor those that you decide to keep.