United States president Barack Obama says a missile defence system covering all NATO countries has been agreed at the alliance's summit in Lisbon.
The shield would cover all 28 members of the North Atlantic Treaty Organisation in Europe and North America. Russia will be asked to join the defence system.
The decision to go ahead with an expanded missile defence system to cover all member countries is a response to the spread of ballistic missiles capable of reaching alliance territory, the BBC reports.
In Washington's view, the primary threat comes from Iran.
But Turkey, which may host a crucial radar station, insisted that it would not back the plan if there was any explicit mention of Iran.
This is only a pointer to some of the problems that may lie ahead - issues of command and control, data exchange and the cost of a truly comprehensive system.
NATO is also backing the swift ratification of the Start Treaty between the US and Russia, cutting nuclear weapon stockpiles.
The summit will discuss Afghanistan on Saturday, with plans to end combat operations by 2014. The US has announced it is to send tanks there for the first time.