by Paul Reynolds BBC World Affairs correspondent

The signs are that President Bush has taken a decision in principle to try to topple Iraqi leader Saddam Hussein - the man his father left in power at the end of the Gulf War. The means, and timing, are as yet unclear.

But the options run all the way to a full-scale invasion. However, options are not decisions.

Mr Bush's own words, decoded, suggest that removing Saddam Hussein is now an aim, not just a wish.

At the mimimum about 200,000 troops would be needed (for an invasion) - and aircraft by the hundred

"Make no mistake about it. If we need to, we will take necessary action to defend the American people," he said at the White House on 13 February.

The phrase "defend the American people" could really mean "attack Iraq".

US Secretary of State Colin Powell has been even more specific. He told a Congressional committee that Washington wanted a "regime change" in Baghdad.

This kind of talk shows that the Bush team has moved on from a decision it took at Camp David on the Sunday after 11 September.

The decision then was to put Iraq on hold for a Phase Two of the war. Afghanistan was Phase One. That has now been largely completed.

So Iraq's time has come.

'Defensive strikes'

The tripwire could come in May. That is when the Security Council is due to renew sanctions against Iraq and that is when the United States, probably supported by Britain, might issue an ultimatum to Iraq to allow UN weapons inspectors in again.

Air exclusion zones have been enforced over Iraq since 1991

If it does not, a reason to take action would then exist. The United States would argue that Saddam is developing weapons of mass destruction and that pre-emptive military strikes against him would therefore be defensive.

What are the options?

Diplomatic and economic measures will continue. But these methods have failed and the Iraqi leader is still there, even though Mr Bush senior departed long ago. The Iraqi people suffer from sanctions, but not, it seems, the Iraqi leadership. It is possible that Iraq might comply and allow inspectors in again, but not probable.

It is likely that President Bush would want to exhaust all other options before deciding on an invasion.

Indirect or covert action is likely - the so-called Northern Alliance approach after what happened in Afghanistan. The CIA is believed ot have been tasked with working up a plan to support opposition groups, although the main one - the Iraqi National Congress - is not regarded very highly by the Americans.

But events have a habit of developing a momentum and if the CIA did organise some low-level action, it could escalate into large-scale defections in the Iraqi armed forces. That would be the hope at least. It could be accompanied by bombing of key Iraqi targets. And maybe feelers would go out to Iraqi officers who might mount a coup.

There could be a bombing only strategy. The United States has developed such accurate weaponry that its planners have far more options than even in the Gulf War. Military targets like research institutions and military bases could be chosen, in order to degrade or destroy Saddam Hussein's arsenal.

Last resort?

But this might not topple him. Dictators under attack from outside have a habit of surviving.

Saddam Hussein remains in power despite military defeat in 1991

This leads inexorably, therefore, to the need for planning for the Big One - an invasion.

The numbers would be huge. At the mimimum about 200,000 troops would be needed - and aircraft by the hundred. And where would they launch from? The US Marine Corps could do it from the sea but on land Kuwait seems to be the only place if Saudi Arabia says no, as it well might.

It would be a vast undertaking and Saddam Hussein might act first, invading Kuwait again maybe and attacking Israel for certain.

It is likely that President Bush would want to exhaust all other options before deciding on an invasion.

But do not rule one out if all else fails.