German Defense Minister Ursula von der Leyen meets German soldiers at Camp Novo Selo in Kosovo, on May 15, 2014 Hazir Reka/Reuters

The Secretary General of NATO touched on one of the more sensitive aspects of the ongoing crisis in Ukraine during a speech at a security conference in Slovakia today. Because Europe has been fairly peaceful in recent decades, and because European NATO states are under the American defense umbrella, they don't need to dedicate all that much of their resources to their own security. This is partly a legacy of the Cold War period, when the U.S. subsidized the defense of European "frontline states" that were potentially threatened by Soviet aggression.

But this dependence on the U.S., along with 20 fairly uneventful years in Europe following the breakup of the Soviet Union, leaves much of the alliance vulnerable to a newly-aggressive Russia — and it might not always be the case that the U.S. is in a position to defend them. Per the AP, NATO head Anders Fogh Rasmussen warned that European countries now risk becoming "free-riders" and told reporters “every ally is expected to play its part toward contributing to our shared security," adding, "we have to invest more in defense.”

According to NATO'S latest annual report on its members' defense spending, the U.S. and Canada spend nearly three times more on defense than all European members combined.

Meanwhile, defense spending actually declined in most NATO states in 2013 (the column on the far right) — even though Europe's biggest security crisis in decades would turn out to be just a few months away.

Rasmussen's statements underscore the long-term shift that Europe's security environment may currently be undergoing. The threats to the continent's stability are growing — and defense budgets, as well as European countries' role in their own security, may have to grow with it.