Given our fragile global order, it’s impossible not to wonder whether the concept of “collective security” has died. The answer depends on what we mean by the term. If collective security means a system in which the world’s major powers renounce using force to alter the status quo and agree to unite to stop any country that violates this pledge, then it is not dead for one simple reason: It was never alive.

The traditional version of collective security—best illustrated by the League of Nations founded after World War I—seeks to transcend power politics by committing states to settle their differences peacefully and to work together to stop any country that violates this principle. Unfortunately, this assumes that dangerous aggressors will be easy to identify and that all the other states will agree on who they are. It further assumes that all the major powers will be willing to act together to stop a powerful aggressor—which is costly and dangerous—even when their own interests are not directly involved. Inevitably, some will be tempted to stand aside and let others deal with the problem. This vision of collective security depends, in short, on a level of trust and selflessness that is rare to non-existent in world politics.

Given our fragile global order, it’s impossible not to wonder whether the concept of “collective security” has died. The answer depends on what we mean by the term. If collective security means a system in which the world’s major powers renounce using force to alter the status quo and agree to unite to stop any country that violates this pledge, then it is not dead for one simple reason: It was never alive.

The traditional version of collective security—best illustrated by the League of Nations founded after World War I—seeks to transcend power politics by committing states to settle their differences peacefully and to work together to stop any country that violates this principle. Unfortunately, this assumes that dangerous aggressors will be easy to identify and that all the other states will agree on who they are. It further assumes that all the major powers will be willing to act together to stop a powerful aggressor—which is costly and dangerous—even when their own interests are not directly involved. Inevitably, some will be tempted to stand aside and let others deal with the problem. This vision of collective security depends, in short, on a level of trust and selflessness that is rare to non-existent in world politics.

But there is another way of defining collective security—namely, as agreements intended to make war less likely or as military alliances where a subset of states join forces to deter or defeat a common threat. History offers many such examples. Unfortunately, even these more modest forms of collective security are not that effective and are becoming less so, which will make the world of the future more dangerous than the recent past.

Even so, the relatively realistic collective security arrangements that have existed in the past deserve deeper consideration today—if only to understand why they are now on life support.

One limited form of a collective security arrangement is a “security regime,” where rivals agree to limit their competition in narrow and specific ways. Arms control agreements such as the SALT and START treaties are good examples, and some of these measures did help reduce........

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