When Nato invokes Article V, it is more than just one member asking for support from the other member states, but Article V invocation means ALL member states view the attack as an attack on them ALL. In other words, NATO Aliance is at war with those responsible, and will support the US lead, which will now hold not only the terrorists responsible, but hold those nations that host them as equally responsible.
"BRUSSELS, Belgium (CNN) -- NATO has unanimously declared the hijack attacks on the U.S. to be an assault against all member states."
"It is the first time in 52 years that the alliance has invoked Article V -- the NATO self-defence charter that says if one member state is under attack all other member nations would defend it."
"The secretary-general added: "The (NATO) Council agreed that if it is determined that this was an attack directed from abroad against the United States, it shall be regarded as an action covered by Article V of the Washington Treaty, which states that an attack against one ally is an attack against them all."
http://www.cnn.com/2001/WORLD/europe/09/12/nato.us/index.html