That question is relevant to Australia, given its location in the "arc of instability", with the threat of failed or failing states on our doorstep. What if the government of a neighbouring state such as Solomon Islands, or East Timor, collapsed and that state became a base for terrorists intent on launching attacks on Australian territory? Or what if a friendly neighbouring country was unable to prevent terrorists from attacking, despite its best efforts? Would these circumstances entitle Australia to invade that country for the purpose of rooting out the terrorists and destroying their bases?

For many years Turkey has been the target of terrorist attacks by the Kurdistan Workers Party (PKK) rebels based in northern Iraq. In the past few months, those attacks have been stepped up considerably, with more than 44 Turks killed in October. Turkey has repeatedly demanded that the Iraqi Government stop the attacks, and extradite the PKK leaders. The United States has also called on the Iraqi Government to stop the attacks, but the reality is that the Government has very little control over northern Iraq. It has disassociated itself from the attacks, condemned them, and ordered the PKK rebels to lay down their arms or leave Iraq. There is, however little evidence that it has the power to enforce these demands.

The prospect of a military invasion of northern Iraq appears increasingly likely. Just last weekend Turkish helicopters fired rockets at Kurdish rebels in Turkey and near the Iraq border, and the rhetoric of the Turkish Government is sounding increasingly belligerent. Turkey has stationed 10,000 troops on the border, ready to invade.

But leaving to one side the disastrous political implications of a military conflagration in north Iraq is this legal? States have certain obligations to each other under international law. A state must not knowingly allow its territory to be used for acts contrary to the rights of other states. It must do all that it can to prevent terrorist planning or training within its territory. It must not support terrorism within its territory either actively, by providing financial or logistic support, or passively, by tolerating or acquiescing in the operation of terrorist bases within its territory. In the post-September 11 world it is clear that a state such as Iraq which is aware that terrorists are launching cross-border attacks from within its territory is in breach of those obligations, arguably even if it lacks the capacity to prevent them.

But what are the consequences of such a breach ? Does it permit Turkey to invade northern Iraq?

The usual consequence of a breach of an international obligation is that the offending state must pay reparation to the victim state, which may also be permitted to impose countermeasures for example a severance of diplomatic relations. Does the breach by Iraq of its obligations to prevent terrorist attacks on Turkey entitle Turkey to respond with force?

The answer is a qualified maybe. The issue has been a topic of hot debate for some time, and especially since the attacks of September 11, 2001. Certainly there was almost universal international consensus as to the US's right to respond in self-defence to those attacks by launching Operation Enduring Freedom, an attack on al-Qaeda bases in Afghanistan. But in that case there was clear evidence that the Taliban, the effective government, had acquiesced in and assisted al-Qaeda's operation of terrorist bases, and had refused requests to shut them down.

Where a state is unable, despite its best efforts, to prevent terrorists from launching attacks from its territory on neighbouring states, it is not clear whether the victim state may take the law into its own hands. Clearly it would be preferable for the matter to be brought before the Security Council, and for it to authorise collective action under Chapter VII of the UN Charter. But that is not always possible, for practical or political reasons.

Some judges of the International Court of Justice have recently argued that a victim state (such as Turkey) can respond with the use of force in self-defence, provided its response is necessary and proportionate the twin requirements of a lawful exercise of that right. Other judges refused to go that far.

Certainly questions arise as to the extent of the force that may be used, and the proper targets questions which arose also in response to Operation Enduring Freedom. To what extent, for example, may the victim state go beyond the destruction of terrorist bases and attack the host state and its infrastructure as well? At what point must it withdraw?

Australia should watch with interest the events as they unfold in Turkey and Iraq these may be issues which Australia has to confront in the future.

Nicole Abadee is a barrister and lecturer in the use of armed force at the University of Sydney law school.