NATO: Under the North Atlantic Treaty, NATO member-states are obliged to regard an armed attack on one of their number as an attack on them all.
Each country promises to "assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area".
Last month, the United States asked NATO's decision-making body, the North Atlantic Council, to draw up plans to help Turkey, which is a member of NATO, to improve its defences against a possible attack from its neighbour, Iraq.
All but three countries - France, Belgium and Germany - agreed that Turkey should receive AWACS surveillance plans, Patriot missiles to repel Iraq's Scud missiles and anti-chemical and anti-biological warfare teams.
The three dissenting countries said that, although they would come to Turkey's aid if it was threatened, sending help now would make war against Iraq appear inevitable.
NATO's Secretary-General, Lord Robertson, said that the plan to help Turkey would go ahead unless one or more member-states objected by yesterday. When France and Belgium issued formal objections, Turkey invoked - for the first time in NATO's history - Article IV of the North Atlantic Treaty.
It states: "The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened".
NATO's 19 nations are the United States, Canada, Belgium, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Britain, Greece, Turkey, Germany, Spain, Poland, Hungary and Czech Republic.
Invited to join in 2004 are Slovenia, Slovakia, Estonia, Latvia, Lithuania, Romania and Bulgaria.
The North Atlantic Treaty was signed on April 4th, 1949 NATO. The alliance went to war for the first time in 1999, intervening in the Yugoslav province of Kosovo.
- Denis Staunton