Russia's offensive in Georgia in August exposed the dangers of the Western alliance's lack of contingency plans against an invasion on its eastern flanks.
Political leaders from the Baltics and Eastern Europe have subsequently demanded that Nato fulfil the requirements implied by its "Chapter 5" commitment to defend the territorial integrity of all its members.
General James Craddock, Nato's Supreme Commander, has asked for the political authority to draw up contingency defence plans at a Nato meeting in Budapest later this week.
France and Germany have signalled opposition to the move but Gen Craddock has the strong backing of American and Britain.
But even US officials acknowledge there is a risk that the move will cause a rift within Nato. "This becomes politicised very quickly," a Pentagon official said.