Former American secretary of state Hillary Clinton warned Western allies on Wednesday that NATO might “slide into military irrelevance” if European member states continue to scale back their defense capabilities.

NATO’s relevance, however, is dependent more upon circumstance and geography than budgets. The United States’ expectations may be unrealistic.

Clinton, who resigned as America’s chief diplomat in February, observed during an Atlantic Council dinner in Washington DC that “NATO is turning into a two-tiered alliance with shrinking percentage of members willing, and able, to pay the price and bear the burdens of common defense.”

She cited NATO’s 2011 intervention in Libya’s civil war. “We saw that fewer than a third of NATO members participated in strike missions.” More countries were willing to help, she said, but lacked the intelligence, surveillance and reconnaissance assets to play a role.

Robert Gates, who was defense secretary when the United States agreed to support an Anglo-French initiative to intervene in Libya, criticized European countries’ defense ineptitude more sharply. Eleven weeks into the operation, he complained that America’s allies were running “short of munitions,” requiring it, “once more, to make up the difference.”

Germany, Poland and Turkey didn’t participate in the Libyan effort at all. Belgium, Canada, Denmark, the Netherlands, Norway and Spain provided limited support.

Gates warned that Americans’ “emotional and historical attachment” to NATO, strong during the Cold War when the West was united against the Soviet Union, was aging out. Future leaders, he predicted, “may not consider the return on America’s investment in NATO worth the cost.”

Only four European NATO members spend more than 2 percent of their gross domestic products on defense, as required by treaty — Albania, France, Greece and the United Kingdom. British and French budget cuts may reduce the number further.

America’s share of total NATO spending has risen since the end of the Cold War, from roughly 50 percent before the Soviet Union collapsed to more than 75 percent today.

Repeated European defense reductions are in part to blame but so is increased American defense spending after the September 11, 2001 terrorist attacks. The military’s budget grew from $291 billion that year to over $800 billion last year, including financing of the war effort in Afghanistan. The Europeans could not, and had less reason to, keep up.

Moreover, the United States used NATO to cloak their own foreign policy in multilateralism which put an unreasonable burden on the European allies. NATO is less a tool of military adventurism for them than a guarantor of security in Europe, especially for the former communist states in Eastern Europe who regard warily Russian interference in the region as well as neighboring Germany’s seemingly improving relations with Moscow.

Clinton’s reference to NATO’s “common defense” is problematic. As European states’ different relations with Russia show, members don’t always agree what constitute common interests and threats.

When they do, as was the case in the Balkans in the 1990s and with counterpiracy operations off the Horn of Africa today, the alliance presents a unified front and all pitch in. When there is disagreement, as was the case in Libya, NATO can still be useful in that a “coalition of the willing” within the alliance benefits from harmonized command and rank structures and the interoperability of military systems.

A “two-tiered alliance” may emerge but only on occasions. Internally, there is little debate about NATO’s security monopoly in the North Atlantic and the role it plays in keeping Europe safe. All member states are willing to contribute to that. Admittedly, they can afford to contribute far less, in relative terms, than during the Cold War.

There is only friction when NATO operates out of theater where it is unreasonable the the United States to expect to have an alliance armed and ready to intervene wherever it believes Western nations have “common” interests. That doesn’t mean NATO is “sliding into irrelevance.” It means NATO’s relevance is geographically limited — as it was from the start.

The rest of NATO, then, hasn’t changed. Its capabilities have simply adjusted to changing circumstances. There are no Russian tanks on the border anymore so why should the Europeans keep theirs? It’s America’s expectation that the alliance would morph into a global police force that has made it seem less effective.