WASHINGTON: “We are of the opinion that the Iranian regime is a rational actor,” said Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, last Sunday on CNN. That sounds right to me, but his comment raises a tricky question: How much pressure will it take to get this “rational” country to curb its nuclear program?

The answer here isn’t comforting: Recent history shows that the Iranian regime will change behavior only if confronted with overwhelming force and the prospect of an unwinnable war. Short of that, the Iranians seem ready to cruise along on the brink, expecting that the other side will steer away.

I count two clear instances when Iran has backed down, and two more “maybes.” These examples remind us that the Iranian leaders aren’t irrational madmen —and also that they drive a hard bargain. Here are the two documented retreats:

— Ayatollah Ruhollah Kho-meini in July 1988 “drank the cup of poison,” as he put it, and agreed to end the Iraq-Iran war. He accepted a UN-sponsored truce, but only after eight years of brutal fighting, Iraqi rocket attacks on Iranian cities, and the use of poison gas against Iranian troops. Khomeini’s decision followed the shoot-down of an Iranian civilian airliner on July 3 by the USS Vincennes—unintended, but a demonstration of overwhelming American firepower in the Persian Gulf.

— Ayatollah Ali Khamenei’s regime halted its nuclear weapons program in the fall of 2003 because of “international pressure,” according to a 2007 US National Intelligence Estimate. The decision came after the March 2003 US invasion of Iraq, which the Iranians apparently feared was the prelude to an attack on their soil. The Iranians also agreed in 2003 to start talks with European nations on limiting their enrichment of uranium — beginning the haggling that continues to this day.

Two other examples are less obvious, but they illustrate the same theme of rational Iranian response to pressure. In both cases the trigger was a strong back-channel message from the United States:

— In March 2008, Iran restrained its Shiite allies in Iraq after a US warning about shelling the Green Zone. The Mahdi Army had been firing heavy rockets and mortars into the enclave, causing rising US casualties. Gen. David Petraeus, then US commander in Baghdad, sent a message—“Stop shooting at the Green Zone”—to Gen. Qassem Soleimani, head of the Quds Force. The intermediary was Iraqi President Jalal Talabani, who had close relations with both generals. The shelling tapered off.

— Last month, Iran toned down its threats to close the Strait of Hormuz after a US back-channel warning that any such action would trigger a punishing US response. The private message paralleled a public U.S. statement: “The United States and the international community have a strong interest in the free flow of commerce and freedom of navigation in all international waterways.” Iranian Foreign Minister Ali Akbar Salehi subsequently offered reassurance: “Iran has never in its history tried to prevent, to put any obstacles in the way of this important maritime route.”

The Iranians’ behavior in negotiations, too, has seemed to wax and wane based on their perception of the West’s seriousness. When Russia and China supported UN sanctions in 2010, the Iranians got nervous. When India and China reduced oil purchases recently, Tehran took notice.

Clear messaging to Iran—and to Israel, too—is important as the tension mounts over a possible Israeli attack on Iranian nuclear targets. The most direct public message yet came from Dempsey in his appearance on Fareed Zakaria’s show, “GPS.” It’s worth looking carefully at just what the nation’s top military officer said.

“The Iranian regime has not decided that they will embark on the effort to weaponize their nuclear capability,” Dempsey said, thereby offering Tehran a chance to save face in any deal. He argued that because Iran isn’t yet building a weapon, it would be “premature” and “not prudent” for Israel to attack. “A strike at this time would be destabilizing and wouldn’t achieve their long-term objectives,” he cautioned. But he conceded that the US hasn’t yet persuaded Israel to hold off.

The signal to Israel is very clear: Don’t attack! But what about the message to Iran? History shows that the clerics in Tehran won’t accept a deal unless they conclude there’s no alternative but a punishing war. Somehow, the US must convince Iran this confrontation is deadly seriou—and then work to find the rational pathway toward agreement.—(c) 2012, The Washington Post Writers Group