Defense Secretary Leon Panetta has a lot on his mind these days, from cutting the defense budget to managing the drawdown of U.S. forces in Afghanistan. But his biggest worry is the growing possibility that Israel will attack Iran over the next few months.

Panetta believes there is a strong likelihood that Israel will strike Iran in April, May or June — before Iran enters what Israelis described as a “zone of immunity” to commence building a nuclear bomb. Very soon, the Israelis fear, the Iranians will have stored enough enriched uranium in deep underground facilities to make a weapon — and only the United States could then stop them militarily.

Israeli Prime Minister Benjamin Netanyahu doesn’t want to leave the fate of Israel dependent on American action, which would be triggered by intelligence that Iran is building a bomb, which it hasn’t done yet.

Israeli Defense Minister Ehud Barak may have signaled the prospect of an Israeli attack soon when he asked last month to postpone a planned U.S.-Israel military exercise that would culminate in a live-fire phase in May. Barak apologized that Israel couldn’t devote the resources to the annual exercise this spring.

President Obama and Panetta are said to have cautioned the Israelis that the United States opposes an attack, believing that it would derail an increasingly successful international economic sanctions program and other non-military efforts to stop Iran from crossing the threshold. But the White House hasn’t yet decided precisely how the United States would respond if the Israelis do attack.

The Obama administration is conducting intense discussions about what an Israeli attack would mean for the United States: whether Iran would target U.S. ships in the region or try to close the Strait of Hormuz; and what effect the conflict and a likely spike in oil prices would have on the fragile global economy.

The administration appears to favor staying out of the conflict unless Iran hits U.S. assets, which would trigger a strong U.S. response.

This U.S. policy — signaling that Israel is acting on its own — might open a breach like the one in 1956, when President Dwight Eisenhower condemned an Israeli-European attack on the Suez Canal. Complicating matters is the 2012 presidential campaign, which has Republicans candidates clamoring for stronger U.S. support of Israel.

Administration officials caution that Tehran shouldn’t misunderstand: The United States has a 60-year commitment to Israeli security, and if Israel’s population centers were hit, the United States could feel obligated to come to Israel’s defense.

Israelis are said to believe that a military strike could be limited and contained. They would bomb the uranium-enrichment facility at Natanz and other targets; an attack on the buried enrichment facility at Qom would be harder from the air. Iranians would retaliate, but Israelis doubt that the action would be an overwhelming barrage, with rockets from Hezbollah forces in Lebanon. One Israeli estimate is that the Jewish state might have to absorb 500 casualties.

Israelis point to Syria’s lack of response to an Israeli attack on a nuclear reactor there in 2007. Iranians might show similar restraint, because of fear the regime would be endangered by all-out war. Some Israelis have also likened a strike on Iran to the 1976 hostage-rescue raid on Entebbe, Uganda, which was followed by a change of regime in that country.

Israeli leaders are said to accept, and even welcome, the prospect of going it alone and demonstrating their resolve at a time when their security is undermined by the Arab Spring.

“You stay to the side, and let us do it,” one Israeli official is said to have advised the United States. A “short-war” scenario assumes five days or so of limited Israeli strikes, followed by a U.N.-brokered cease-fire. The Israelis are said to recognize that damage to the nuclear program might be modest, requiring another strike in a few years.

U.S. officials see two possible ways to dissuade the Israelis from such an attack: Tehran could finally open serious negotiations for a formula to verifiably guarantee that its nuclear program will remain a civilian one; or the United States could step up its covert actions to degrade the program so much that Israelis would decide that military action wasn’t necessary.

U.S. officials don’t think that Netanyahu has made a final decision to attack, and they note that top Israeli intelligence officials remain skeptical of the project. But senior Americans doubt that the Israelis are bluffing. They’re worrying about the guns of spring — and the unintended consequences.

davidignatius@washpost.com