Just four years ago, Iran seemed to be consolidating and even expanding its position in the Middle East. Its proxies in Lebanon, Palestine, Syria, and Yemen had established themselves as organized military forces in their respective regions. Iran’s missile program appeared to be improving rapidly in both scale and technological sophistication. Meanwhile, Iran had increased uranium enrichment to 60 percent, bringing it within reach of weapons-grade levels should it decide to make that leap.

Today, the Islamic Republic is fighting for its survival. Israeli and U.S. warplanes are striking deep into its territory, dismantling the military that Iran has spent the past decade building. Most of its proxy forces—perhaps with the singular exception of the Houthis—are in shambles. Iran’s surface fleet has largely been sunk, its air force is effectively nonoperational, and its air defenses have proved to be incapable of stopping sustained attacks. The only force still capable of striking regional targets, its missile and drone arsenal, is under continuous bombardment as aircraft bomb the entrances to Iran’s so-called “missile cities.”

Just four years ago, Iran seemed to be consolidating and even expanding its position in the Middle East. Its proxies in Lebanon, Palestine, Syria, and Yemen had established themselves as organized military forces in their respective regions. Iran’s missile program appeared to be improving rapidly in both scale and technological sophistication. Meanwhile, Iran had increased uranium enrichment to 60 percent, bringing it within reach of weapons-grade levels should it decide to make that leap.

Today, the Islamic Republic is fighting for its survival. Israeli and U.S. warplanes are striking deep into its territory, dismantling the military that Iran has spent the past decade building. Most of its proxy forces—perhaps with the singular exception of the Houthis—are in shambles. Iran’s surface fleet has largely been sunk, its air force is effectively nonoperational, and its air defenses have proved to be incapable of stopping sustained attacks. The only force still capable of striking regional targets, its missile and drone arsenal, is under continuous bombardment as aircraft bomb the entrances to Iran’s so-called “missile cities.”

How did Iran’s position deteriorate so quickly? The reasons are numerous, but a key turning point was the realization by Israel and the United States some two years ago that Iran’s military capabilities, particularly its........

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