Last April, it appeared as though escalation between Israel and Iran could plunge the entire Middle East into conflict. Israel’s strikes on the Iranian consulate in Damascus prompted Iran to retaliate by launching a barrage of missiles and rockets into Israel—the first time that Iran had openly attacked the country. But after Israel responded in a relatively muted way, both countries moved on from the confrontation. Observers, too, put aside their most acute worries, comforted by the fact that both countries had shown that they had no interest in a wider war.

This conclusion, however, was premature. In September, Israel intensified its campaign against Hezbollah, the Iranian-backed paramilitary group operating in Lebanon. This marked an important shift: it suggests that Israeli leaders decided they wanted to actively reshape the balance of power in the Middle East. Much more than its actions in Gaza, Israel’s war against Hezbollah threatens Iran’s ability to project power and profoundly diminishes its ability to deter Israeli interventions into its own domestic politics and nuclear program. The weakening of Iran’s position will benefit Israelis in the short term. But in the long term, it will significantly increase the risk of a regional war and even the likelihood that Iran will acquire nuclear weapons. To avoid being dragged into yet more conflict in the Middle East, the United States must work to restrain further Israeli action and stabilize the balance of power.

Deterrence, at its core, is the ability of one actor to alter the cost-benefit calculus of another in order to prevent unwanted actions—usually in an effort to preserve the status quo. Yet deterrence is rarely synonymous with absolute peace; rather, it is about preventing an adversary from crossing specific redlines. During the Cold War, for example, the United States and Soviet Union were highly effective in deterring each other from launching preemptive or direct attacks but still conducted decades’ worth of proxy wars, arms sales to adversaries, and outright invasions of third countries. The United States has been effective thus far in deterring China from invading Taiwan but has not prevented China from undertaking other provocative actions in the South China Sea. The tendency in international politics whereby states are deterred from direct attacks in favor of indirect action is at the heart of the stability-instability paradox theorized in security studies—in which stable nuclear deterrence actually encourages more aggression using conventional military methods—and has been a central part of Iranian strategy in the Middle East.

To that end, deterrence is not only relevant to shaping decisions about whether to act. It also plays........

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