It was a brave decision for President Barack Obama to send 30,000 additional troops to Afghanistan — and the right one. And he defended it eloquently in accepting his Nobel Peace Prize.

[IMGCAP(1)]Now, of course, comes the hard part — not just in Afghanistan and Pakistan, but in U.S. politics and, especially, Iran.

One major virtue of Obama’s decision was to make it clear that, while his foreign policy approach is less bellicose than former President George W. Bush’s, he’s not a pushover for America’s adversaries.

He’s never been the wussy, apologize-for-America weakling portrayed by his right-wing critics, former Vice President Dick Cheney in the lead.

His “apologies— for past U.S. mistakes clearly have been part of a strategy to identify with suspicious audiences — such as Muslims — and he’s invariably followed up with assertions of U.S. policy — such as defenses of Israel — that his audiences didn’t want to hear.

On the other hand, the rulers of Iran, who are getting away with building nuclear weapons and sponsoring terrorism, may well have thought of Obama as another President Jimmy Carter, incapable of effective use of coercion.

Now, he’s demonstrated that he is willing to use force — and intensify it, against domestic opposition — to defend America’s vital national interests. As he said at the Nobel ceremony in Oslo, “there will be times when nations — acting individually or in concert — will find the use of force not only necessary but morally justified.—

“Evil does exist in the world. A nonviolent movement could not have halted Hitler’s armies. Negotiations cannot convince al-Qaida’s leaders to lay down their arms,— he went on. And he concluded: “I, like any head of state, reserve the right to act unilaterally if necessary to defend my nation.—

Because Iran shows no inclination to yield to international demands to stop its nuclear program, the question is: How do Obama’s words resonate in Jerusalem?

Israel regards Iran’s nuclear program as an existential threat. If the international community cannot invent sanctions that will stop that program, by Obama’s own logic, Israel would have every right to attack Iran’s installations.

Obama surely would not want that to happen, but time is running out. He said earlier this year that Iran had until the end of 2009 to come to terms on its nuclear program.

He said specifically in Oslo that “nations like Iran and North Korea— must not be allowed to “game the system.—

“Regimes that break the rules must be held accountable. Sanctions must exact a real price. Intransigence must be met with real pressure — and such pressure exists only when the world stands as one.—

The problem is that the world does not stand as one on any tough sanctions regime — such as, for instance, a cutoff of Iran’s gasoline supplies.

Almost certainly, if Israel asked for Obama’s support for raids on Iran, he’d refuse. The administration seems to believe that Cold War-style deterrence and containment will work on Iran as it did with the Soviet Union.

They might — but the Israelis see Iran as Hitler’s Germany, not as Brezhnev’s Russia. And, if they decide to attack, they will quote Obama at Oslo.

His Oslo speech — and much of his Dec. 1 West Point speech, too — seemed designed to convince his fellow Democrats of the rightness of what he was doing. That explains his plan to begin winding down the Afghanistan surge 18 months from now — and, his differentiation of Afghanistan from Vietnam, the source of Democrats’ chronic force-aversion.

And, briefly, he seems to have convinced them. A Dec. 3 Gallup poll showed that 58 percent of Democrats supported his decision — even more than Republicans, at 55 percent.

But that’s not likely to last. Prior to his West Point speech, a Nov. 25 Gallup poll showed that 57 percent of Democrats wanted to begin reducing U.S. forces and only 29 percent supported an increase.

In the meantime, 72 percent of Republicans supported an increase, including 65 percent who favored adding 40,000 troops. Those numbers are probably the baseline to which public opinion will return as casualties mount.

Significantly, on Tuesday, Obama’s decision got a strong endorsement — and a promise of continuing support — from his 2008 opponent, Sen. John McCain (R-Ariz.), who questioned only Obama’s “exit sign.—

Practically the entire Democratic establishment, including Speaker Nancy Pelosi (Calif.), Senate Armed Services Chairman Carl Levin (Mich.) and even Vice President Joseph Biden, have let it be known they opposed the surge.

Perhaps one good outcome of Obama’s decision is that he will find something that he can establish bipartisanship around. One thing might lead to another, though it’s doubtful.

Afghanistan and Pakistan are tough places where suspicion of the United States runs deep, along with distrust of their own governments.

But Obama has pledged to stand with those populations and help them become more secure and economically developed.

They will believe him — and adversaries such as Iran will believe him — only if he sticks with the task and aims to win.