While Americans are enjoying their summer vacations, the Bush administration is preparing for war. I mean real war, with tens of thousands of troops. President Bush has been shown a military plan to send tens of thousands of troops to Iraq to topple Saddam Hussein.

Pentagon press leaks hint of battle by early next year. The president says repeatedly that his policy on Iraq is “regime change.” Yet the prospect of a war involving up to 250,000 U.S. troops has hardly raised a frisson of public concern.

I think that’s because few people grasp that such a war is really going to happen. The word war is thrown around so loosely these days as in “war on terrorism” or “war on corporate greed” that it has lost much of its power. Polls show a majority of Americans favor sending troops to oust Saddam, but pollees probably thought the question was pro forma.

Well, it isn’t. Which is why the time has come for a national debate over the why and how of war with Iraq.

The president has yet to lay out a full rationale for war to the public or to Congress. The administration remains split over war strategy. The case for going after Saddam can indeed be made, but it requires more than linking Iraq to the “axis of evil.”

No question, Saddam’s evil deeds are legion. He displayed his disregard for international norms by invading Kuwait. He used poison gas on Iraqi Kurds and slaughtered tens of thousands of his people. The vast majority of Iraqis hate him.

But the main threat he poses is his drive to acquire weapons of mass destruction chemical, biological and nuclear and the means to deliver them. Should he succeed, he could dominate and threaten the whole Mideast.

Moreover and this is key no new regime of U.N. weapons inspectors has a chance of finding the programs he is hiding. Saddam endlessly blocked, then expelled the previous U.N. inspectors and would only let in new ones under a mandate so constricted as to be of little value.

Having said all this, there still is no immediate casus belli for a U.S. attack.

The administration has tried but failed to find a “smoking gun” linking Iraq with the Sept. 11 attack or with Osama bin Laden. We can surmise that Saddam might one day hand off WMD to anti-American terrorists. I have my doubts Saddam’s main aim is survival, and I wonder whether he would risk having such a handoff traced back to him. But one cannot discount such a danger.

So the question becomes: Can we afford to wait, or must we act first?

This is not an easy call, and pre-emption is a dangerous concept if adopted as a broader doctrine. But I believe Bush can make the case for pre-emption in Iraq. The decisive factor for me is what would happen if Saddam were allowed to stay in power.

For the past decade the Iraqi leader has been contained by U.N. sanctions that control his oil revenues and prevent him from openly buying arms or building WMD. But these sanctions are crumbling. Many countries, such as France and Russia, are eager to start selling arms again to Baghdad.

If Saddam is left alone, I believe that within five years he will be fully back in business, threatening the whole region and beyond. If he is replaced, even by a half-decent regime, the Mideast will be much freer to focus on development and on finding a solution to the Israeli-Palestinian impasse.

But it’s not enough to make the case for why. The president must also clarify what kind of war he has in mind. Americans and allies need to know whether the Bush team plans to aid Iraqis to liberate themselves or plans a massive invasion and occupation of Iraq.

The former strategy might win tacit acceptance in the region. The latter portends disaster. This undertaking is too serious to be approached without public and congressional airing.

“We need a national dialogue,” says Nebraska Sen. Chuck Hagel, a respected Republican voice on foreign policy. “If the United States decides to take action against Iraq, Americans need to understand the risks and objectives.”

It’s time for the Bush team to make the case for war.

Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.