Relations between Turkey and Israel grow more tense and ugly by the day. There is even growing speculation that the two countries might come to blows. Ankara’s pledge to provide naval escorts for future humanitarian relief flotillas headed for Gaza certainly has the potential to trigger such a conflict.

If that scenario took place, it would put the United States in a horrific dilemma. Although there is no legal obligation to defend Israel, that country has long been Washington’s favorite partner in the region, and it exerts a powerful emotional hold on major portions of the American population. Yet Turkey is a fellow member of NATO, and the United States has an explicit treaty obligation to assist that country if it is attacked.

If fighting erupted between Israeli and Turkish forces and Ankara invoked Article 5 of the North Atlantic Treaty, which proclaims that an attack on one member is considered an attack on all, Washington would face a nightmare scenario. U.S. leaders would then have to decide if Turkey was the victim of aggression or had provoked an attack. The likelihood is that if forced to choose between Israel and Turkey, Washington would find that Turkey was at fault. Regarding Israel as an aggressor whose conduct mandated a U.S. military response is utterly unthinkable, especially given the firestorm such a decision would create at home.

But if the United States reneged on its Article 5 commitment, Turkey would almost certainly withdraw from NATO, and relations between Washington and Ankara would plunge into the deep freeze. Other alliance members would certainly wonder about the reliability of their security commitment under Article 5. In short, the episode could well be the death blow for NATO.

The possibility of an armed clash between Israel and Turkey is not the only instance in which Washington has reason to worry. Throughout the Cold War, U.S. officials worried repeatedly that war might break out between NATO members (and long-time adversaries) Greece and Turkey. That risk reached an extremely dangerous level in 1974 when Turkey invaded Cyprus and expelled the Greek Cypriot population from the northern portion of the island. Although tensions between Athens and Ankara have receded somewhat in the past few years, the possibility of a clash has not disappeared.

On the other side of the world, Japan and South Korea have frequently been at odds—especially regarding a territorial dispute over some small islands (known as Dokdo in Korea and Takeshima in Japan) in the East China Sea. There have been several instances of naval deployments by both countries that could easily have gotten out of hand.

Again, any type of armed encounter between Japan and South Korea would create a huge headache for the United States. Washington has bilateral security treaties with both countries. Which party would the United States defend, if the two nations ever came to blows? (One wag suggested that since the major U.S. military presence in South Korea consists of army units, whereas the principal presence in Japan consists of Marines stationed on Okinawa, the army should fight alongside South Korean forces and the Marines stand shoulder to shoulder with the Japanese.)

There would be little in the way of humor, though, if a conflict erupted between two close U.S. allies. That risk underscores one of the many problems inherent in signing defense treaties or otherwise providing security guarantees to allies and clients. Such commitments pose few problems as long as they’re never challenged. But if one of those security partners comes under attack, whether from a U.S. adversary or another U.S. ally, America is immediately embroiled in a conflict that prudent policy makers might want to avoid. And as the United States adds more and more security clients (as it has done with the expansion of NATO since the mid-1990s), the danger of such a dilemma mounts.