Having leaked to the world, and thus to Syrian President Bashar al-Assad, a detailed briefing of the coming U.S. air attack on Syria — (1) the source (offshore warships and perhaps a bomber or two), (2) the weapon (cruise missiles), (3) the duration (two or three days), (4) the purpose (punishment, not “regime change”) — perhaps we should be publishing the exact time the bombs will fall, lest we disrupt dinner in Damascus.

So much for the element of surprise. Into his third year of dithering, two years after declaring Assad had to go, one year after drawing — then erasing — his own red line on chemical weapons, Barack Obama has been stirred to action.

Or more accurately, shamed into action. Which is the worst possible reason. A president doesn’t commit soldiers to a war for which he has zero enthusiasm. Nor does one go to war for demonstration purposes.

Want to send a message? Call Western Union. A Tomahawk missile is for killing. A serious instrument of war demands a serious purpose.

The purpose can be either punitive or strategic: either a spasm of conscience that will inflame our opponents yet leave not a trace, or a considered application of abundant American power to alter the strategic equation that is now heavily favoring our worst enemies in the heart of the Middle East.

There are risks to any attack. Blowback terror from Syria and its terrorist allies. Threatened retaliation by Iran or Hezbollah on Israel — that could lead to a guns-of-August regional conflagration. Moreover, a mere punitive pinprick after which Assad emerges from the smoke intact and emboldened would demonstrate nothing but U.S. weakness and ineffectiveness.

In 1998, after al-Qaeda blew up two U.S. embassies in Africa, Bill Clinton lobbed a few cruise missiles into empty tents in Afghanistan. That showed ’em.

It did. It showed terminal unseriousness. Al-Qaeda got the message. Two years later, the USS Cole. A year after that, 9/11.

Yet even Clinton gathered the wherewithal to launch a sustained air campaign against Serbia. That wasn’t a mere message. That was a military strategy designed to stop the Serbs from ravaging Kosovo. It succeeded.

If Obama is planning a message-sending three-day attack, preceded by leaks telling the Syrians to move their important military assets to safety, better that he do nothing. Why run the considerable risk if nothing important is changed?

The only defensible action would be an attack with a strategic purpose, a sustained campaign aimed at changing the balance of forces by removing the Syrian regime’s decisive military advantage — air power.

Of Assad’s 20 air bases, notes retired Gen. Jack Keane, six are primary. Attack them: the runways, the fighters, the helicopters, the fuel depots, the nearby command structures. Render them inoperable.

We don’t need to take down Syria’s air defense system, as we did in Libya. To disable air power, we can use standoff systems — cruise missiles fired from ships offshore and from aircraft loaded with long-range, smart munitions that need not overfly Syrian territory.

Depriving Assad of his total control of the air and making resupply from Iran and Russia far more difficult would alter the course of the war. That is a serious purpose.

Would the American people support it? They are justifiably war-weary and want no part of this conflict. And why should they? In three years, Obama has done nothing to prepare the country for such a serious engagement. Not one speech. No explanation of what’s at stake.

On the contrary. Last year Obama told us repeatedly that the tide of war is receding. This year, he grandly declared that the entire war on terror “must end.” If he wants Tomahawks to fly, he’d better have a good reason, tell it to the American people and get the support of their representatives in Congress, the way George W. Bush did for both the Afghanistan and Iraq wars.

It’s rather shameful that while the British prime minister recalled Parliament to debate possible airstrikes — late Thursday, Parliament actually voted down British participation — Obama has made not a gesture in that direction.

If you are going to do this, Mr. President, do it constitutionally. And seriously. This is not about you and your conscience. It’s about applying American power to do precisely what you now deny this is about — helping Assad go, as you told the world he must.

Otherwise, just send Assad a text message. You might incur a roaming charge, but it’s still cheaper than a three-day, highly telegraphed, perfectly useless demonstration strike.

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