SINGAPORE (Reuters) - It sounds like an arcane debate among wordsmiths. But the distinction between pre-emptive and preventive war, drawn by President Bush in ordering the U.S. invasion of Iraq, could change the face of war.

Galvanized since the September 11 attacks by a need to protect the homeland, Bush has tossed aside, if not quite torn up, the U.N. Charter on war.

Strict conditions exist to undertake pre-emptive war and Bush has bypassed those to launch a preventive war, analysts say.

In simple terms: imagine a row with your neighbor over an overhanging branch. You see him advancing on the bough with his buzzsaw running. You may pre-empt his attack.

But if you just suspect he's been to the hardware store to buy a saw, you may not burn down his garden shed to prevent him taking a slice out of the disputed greenery.

Bush's preventive action is an innovation in contemporary history and opens the way for others to follow suit.

"While it is not true that the U.S. has been able to establish a new norm of prevention, other expedient states may use the U.S. action as justification, even though they are likely to be roundly condemned," said Chris Reus-Smidt of the Department of International Relations at the Australian National University in Canberra.

WHO WOULD DARE?

There is reason to doubt whether any state would have the courage to take on such condemnation. But many may feel they are next in the firing line.

Thus the ramifications are far-reaching, not just for countries with perceived enemies on their borders such as India and Pakistan, but also for Iran and North Korea -- the two nations that Bush bracketed with Iraq in his "axis of evil."

"This is tectonic," said Uday Bhaskar of the Institute of Defense Studies and Analyzes in New Delhi.

"Before March 20 there had been a sense since the end of World War II to the end of the Cold War that a certain consensus existed about the use of force and how that should be regulated.

"It sets a precedent," he said. "This is a threat to stability, an action that induces anxiety. The question is why can't it be a France next time, or an India?"

Analysts fear that the period of relative peace since the birth of the United Nations after World War II, with its strict charter injunction against the use of force, could now be in serious jeopardy.

"The doctrine of pre-emptive war has profoundly destabilizing implications for international society," said Reus-Smidt.

"The legal restriction of the use of force to unequivocal acts of self-defense and international peace enforcement actions is one of the principal reasons for the radical decline in interstate wars, even as the number of states has multiplied."

WHERE ARE THE LIMITS?

Few nations have flouted the U.N. charter that lays out specific conditions for the use of pre-emptive force. Two extraordinary exceptions are Israel's 1981 strike on Iraq's OsIrak nuclear plant and the 1967 Six Day War, said Reus-Smidt.

"The major innovation of the Bush doctrine is the idea of prevention, and the war in Iraq can be seen as the first example of this," said Reus-Smidt.

He said Washington, rebuffed in the U.N. Security Council in its quest for world backing to pre-empt Saddam Hussein's suspected weapons program, had opted to act preventively.

That opens a Pandora's Box.

"It's not clear what the limits are," said Hilary Charlesworth, professor at the Center for International and Public Law at the ANU.

"This leaves the perception of threat in the eye of the beholder."

It reinforces fears of the United States going it alone, snubbing the international community when it suits it, for example on the Kyoto treaty on global warming or the International Criminal Court.

The United States has acted as other countries have throughout history, which is to look for the international law that suits them. And it was that free-for-all approach that the U.N. charter was aimed at halting.

"We could be going back to a pre-U.N. charter world and I find that worrying," said Charlesworth.

Of course, what goes unspoken is that the United States regards itself as an exception, and knows that it can probably get away with a preventive war because it has more toys, and more powerful ones, than anyone else in the playground.

MORE SACRED THAN OTHERS

"The related political and diplomatic question is 'are we redefining sovereignty?"' said Bhaskar. "It's an Orwellian kind of sovereignty in which some are more sacred than others."

Analysts believe that deterrence may work in this new world, and thus a nuclear-ambitious North Korea may not be next. But what, asked one, would stop China taking a swipe at Taiwan?

"What will be the restraints?" said Charlesworth. "International law is enforced by a sense of reciprocity and this is doing away with the fabric of international law."

Some say international law may have to change to ensure relevance in a world threatened by rogue states and suicide hijackers.

When Osama bin Laden's Islamic revolutionaries flew planes into the World Trade Center, they may not only have transformed the course of history, but have wrought upheaval in the rules of war.

"After September 11, in a world in which unprovoked acts of terrorism could cost hundreds of thousands of lives, deterrence and passive self-defense are not enough," The Australian newspaper wrote in an editorial on Thursday.

That is a view that may hold sway at the Pentagon and in the White House, but stirs anxiety among legal and defense experts.

"What is sauce for the goose is sauce for the gander," said Bhaskar. While other countries may lack the will, or indeed the might, to follow in U.S. footsteps they may be sorely tempted.

"The net effect of all of this is that it greatly increases the risk of wars, preventive and pre-emptive," said Reus-Smidt.