In 2006, the 28 members of NATO agreed to spend at least 2% of their GDP on defense. According to NATO records, by 2012 only four members met this bare minimum standard; the United States, Great Britain, Greece, and Estonia. During the Cold War, the U.S. accounted for roughly 50% of defense spending by NATO members. Now after years of shrinking defense budgets in Europe, the U.S. share is more than 75%.

It is a priority for both President Obama and Hagel to convince our allies to take specific actions to fulfill their alliance commitments. This point will be communicated to them this week and at every top NATO meeting leading up to the summit next year in London. This is not an issue the Obama administration can walk away from. In fact, criticism of the excessive dependence on U.S. capabilities by NATO allies is one of the few issues that enjoys widespread bipartisan support in Congress. If U.S. allies continue to ignore the gravity of this problem, it is inevitable that the day will come when the United States will stop payment on their security credit card.

The Obama administration began moving in this direction when it chose to “lead from behind” in Libya and limited the use of important U.S. enablers such as unmanned aircraft and A-10 war planes. The administration already crossed the threshold, but changed its mind, when it initially asked the French military to pay for the use of U.S. air transports for the French mission in Mali.

The European members of NATO are geographically closer and arguably more vulnerable than the U.S. to the growing violence and instability across the Mediterranean, the Sahel, and the Middle East. Unless significant progress is made to fairly live up to their defense commitments, Europe will have to deal with these threats with a decreasing amount of U.S. military support. It is only a matter of time before the Obama administration turns down future requests for assistance from allies who ignored the dangers in their neighborhood and chose to starve their defense capabilities.

Unless our European allies change course, Gates’ dire warning will soon come true. "The blunt reality is that there will be dwindling appetite and patience in the U.S. ... to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defense."

This can still be avoided if our NATO allies improve their military capabilities and carry their fair share of the defense burden. What cannot be avoided is the end of the status quo. Either our European allies change their defense behavior or the U.S. will, but the current imbalance within the NATO alliance is not sustainable.

Dr. Jorge Benitez is the director of NATOSource and a senior fellow at the Atlantic Council.