It may turn out to be the biggest military campaign in Europe since 1945. Russian pressure was ignored as NATO last night went to war without a specific UN mandate.

For the first time since Slobodan Milosevic triggered the break-up of Yugoslavia almost a decade ago, his own sovereign territory will be attacked by the international community. There is another first: German military personnel will be involved in offensive action for the first time since the second World War. The most controversial aspect of the military action now under way is the lack of an explicit UN mandate. Of the five permanent members of the UN Security Council, three (the US, Britain and France) maintain that resolution 1199 on Kosovo approved last September gives a mandate for action. Two (Russia and China) do not.

The Irish Government stubbornly refuses to express a view as to whether there is a mandate for this action. Pointing to the division on the Security Council, the Minister for Foreign Affairs, Mr Andrews, maintained yesterday it was not up to Ireland to interpret the situation for the Security Council.

Our fellow European neutral, Austria, showed no such reluctance yesterday. An Austrian government spokesman explained that his country would refuse to allow NATO planes to use its airspace, as NATO had no UN mandate. Austrian constitutional law dictated it take this stance. Mr Andrews declined to take a clear position on the question of the necessity of a UN mandate last night. He had hoped for a peaceful solution, he said, but that appeared not to be happening and he regretted this.

An attack on a sovereign state over what is officially an internal matter is a dramatic departure by the West. However, the reality is that the Kosovo issue is not internal: the treatment of the Albanian majority in the province by the authorities has potential to destabilise neighbouring states and to create a major international refugee crisis.

For Russia, air strikes on Yugoslavia will be a major diplomatic defeat, although that country is distracted and weakened by its own economic crisis. Last night, the Russian Prime Minister, Mr Yevgeny Primakov, was making yet another "final appeal" to President Milosevic to back down.

However, NATO began its military action despite Russian objections and threats of retaliatory moves. The Russian news agency Tass said yesterday that military staff were considering stationing tactical nuclear weapons in Belarus - which borders new NATO member Poland. The Russian Foreign Minister, Mr Igor Ivanov, reiterated that Moscow could reconsider its co-operation pact with NATO. He also said Russia could reconsider its commitment to an international arms embargo on Yugoslavia.

But after years of watching President Milosevic ignore dozens of "final appeals", the West was determined to act. Having apparently found some determination, however, a question mark hangs over whether the planned strategy will work.

The last Western attack in the region was a remarkable success.

In 1995 NATO planes finally bombed Bosnian Serb military targets after four years of war. Within little more than a week Bosnian Serb army communications and defences were in ruins, the siege of Sarajevo was over and the Bosnian Serbs had agreed to come to the negotiating table to hammer out what became the Dayton Accord.

But the fact that it worked before does not guarantee it will work now. In 1995, the Bosnian Serb army was weakened and demoralised after a series of defeats by the Croats. The Serbs had been driven from the Krajina region of Croatia and were incapable of winning control of any more of Bosnia's territory. A ceasefire and peace suited President Milosevic.

This time around, nobody is sure whether Mr Milosevic wants peace or war. There are two theories as to his game plan: the only thing they have in common is their supreme cynicism.

The first theory is that he decided a considerable time ago he was going to lose Kosovo. The province's population is 90 per cent ethnic Albanian and the Kosovo Liberation Army is maintaining a constant guerrilla campaign. The level of repression needed to keep down the popular demand for independence would ultimately lead to Western military intervention.

In short, according to this theory, Milosevic knew he would lose, but having posed for so long as Serbia's nationalist strong man, he could not simply give up the province without a fight. Thus, this argument goes, he actually wants be confronted by a major NATO offensive so he can say he had no choice but to concede in the face of overwhelming odds.

The second theory is that he believes he can see off NATO. Air strikes may destroy Milosevic's air defences and communications centres, but they will not prevent Milosevic's ground offensive in Kosovo against the Albanian population. NATO will not commit the ground troops that could protect the population, therefore the air strikes will fail, Russian pressure for them to end will increase, NATO resolve will weaken and the crisis will pass. Some "collateral damage" to civilians during the bombing raids would add to the pressure on NATO to abandon its strategy.

Milosevic's preferred outcome may be somewhere in between the above two scenarios - conceding the loss of much of Kosovo but retaining the northern part which contains the region's mines and mineral wealth. His recent offensive has once again led to substantial population movements, and its effect may be to ethnically cleanse this most strategically useful part of Kosovo.

Once this task is completed, he may well agree to talks towards a deal that would recognise the new ethnic make-up of this part of the province.

If Milosevic's strategy isn't known, that of NATO is unclear as well. Mr Tony Blair told the House of Commons on Tuesday the aim of air strikes would be to prevent a "humanitarian disaster" and to protect civilians from "military suppression".

However, to achieve these laudable aims NATO appears to be relying solely on the effects of a massive air bombardment. While this will damage and destroy Yugoslav communications and military equipment, it will not physically prevent Serbian military action in Kosovo, which can continue without high-tech defences.

So the question is: what will happen after the air strikes? The former commander of United Nations forces in Bosnia, Gen Sir Michael Rose, warned on BBC radio earlier this week: "If you carry out any act of war you have to be prepared to go the whole distance. At the moment I doubt if bombing will do the trick . . . You can only do it by putting an army on the ground."

However, there is no intention of putting an army on the ground.

Instead, NATO appears to hope that the air strikes alone will bring Milosevic to the negotiating table, as they did in 1995.

Nobody knows what Plan B is.