Throughout the Cold War, the North Atlantic Treaty Organization (NATO, also known as the North Atlantic Alliance) served as a bulwark against a Soviet invasion of Europe. The core of the treaty is that "the Parties of NATO agreed that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all." This agreement was of the utmost importance to the nations of Europe that, unlike the United States, faced the very real possibility of fifty Soviet and Warsaw Pact armored divisions and nearly two million troops pouring down the North European Plain into the heart of the continent. At one point during the Cold War, the Warsaw Pact countries had a 2-to-1 advantage in main battle tanks. The Soviets and their allies believed that their numerical superiority would allow them to advance to the Rhine River in seven days, and thus Europe was highly dependent on America's promise to retaliate critically as a safeguard against invasion.
Official credos aside, Lord Ismay, NATO's first Secretary General, stated a more pragmatic view of its governing principle: "to keep the Russians out [of Europe], the Americans in, and the Germans down." The U.S. wanted to be "in" because part of its grand strategy has been to ensure that no unified entity arises in Eurasia. If Russia occupied Europe, combining Europe's technology, ports, and other resources with its own, it would be in a good position to threaten America. Lord Ismay's mention of "keep[ing] . . . the Germans down" is also revealing, for NATO—like the European Union project—was designed to keep Germany so economically, politically, and militarily lashed to its neighbors that it would be unable (or at least unwilling) to go to war with them again.
For four tense decades, then, continental Europe depended on the U.S. to deter the Soviet Union. While Europe and America did not agree on various interests and strategies, the threat of Soviet invasion loomed larger than anything else on the horizon. Despite not liking all the arrangements, Europe needed a security guarantee against the Soviets. NATO provided that protection without requiring much in the way of finances (which, incidentally, led to the birth of the European welfare state: Its defense budgets could afford to be much smaller than for a country looking out for its own defense, allowing for increased state-provided services).
However, the geopolitical landscape has undergone substantial change over the last twenty years. With the dissolution of the Warsaw Pact in mid-1991, and the U.S.S.R.'s collapse later that year, the 28-member Alliance has had difficulty defining its reason for existing. With no clear threat, NATO enlargement became a goal in and of itself. It took up the cause of "humanitarian intervention" in the Balkans, and the 9/11 attacks paved the way for NATO involvement in Afghanistan; but neither of these proved to be a unifying threat for the alliance.
In short, without the overwhelming threat posed by the Soviet Union, each member state has begun to develop its own threat priorities. While the U.S. prefers to fight militant Islam on foreign battlefields, most European nations consider terrorism as a domestic law-enforcement issue. With the U.S. tied up in Iraq and Afghanistan, Russia has resurged into much of its former sphere of influence, causing consternation for NATO members in Eastern Europe, but eliciting little response from Western and Southern Europe.
Germany is certainly not interested in another Cold War. Rather than being "down," Germany—again becoming the strongman of Europe—is shaping European policy, which tends to welcome Russian cooperation rather than to view it as a threat. Hence, European NATO allies are divided among themselves in their views on the biggest threat, and thus on the primary reason for the alliance to continue.
Recent events in Europe have shown that the EU is not a completely stable instrument of prosperity, especially where Germany is concerned. Much to its dislike, Germany is being called on to prop up some of the more insolvent members. Rather than pushing for further European integration (which could leave it entangled and unable to protect its own interests), Germany has begun to look outside the Union—and found Russia.
Russia supplies Germany with nearly 40% of its natural gas. Also, Germany's population is declining, while Russia has a surplus population (relative to its labor needs, at least). Moscow needs Western technology and expertise in developing its economy. Thus, German investment in Russia allows Germany to get the labor it needs (without resorting to immigration) by moving production facilities to Russia. Berlin's policy toward Russia is to give it enough of a stake in the European economic system that Russia does not seek to challenge the European security system. The bottom line is that, although NATO proved effective in deterring a Soviet attack, Russia is no longer "out" of Europe, and the Germans have not been kept "down."
In November, the 28 NATO members will meet in Lisbon to approve a new "Strategic Concept," the alliance's mission statement for the next decade. That meeting's outcome, however, already appears to be irrelevant. In late October, France, Germany, and Russia held a summit regarding a Russian proposal for a new European Security Treaty. Even though Moscow claims that the proposal is not intended to replace NATO, the U.S. and the Central Europeans see it as attempting to do exactly that—akin to the fox providing security for the henhouse.
The fact that France and Germany are willing to entertain such a proposal indicates that they no longer see NATO as in their best interests. Not only do they not see Russia as a threat, but having their own security arrangements would also keep them from having to participate in any future expensive "American adventurism," such as the war in Afghanistan.
NATO is still technically viable, but it is becoming more apparent to many of its members that it has outlived its usefulness. We may be watching the beginning of its end.