In Rwanda, we didn't, and 800,000 died.
In Bosnia, we tarried, and more than 100,000 died and 2 million were displaced before we acted.
It's time to take those lessons and now act in Ukraine.
In the Balkans in 1991, we let the Europeans lead with diplomacy to halt Serb aggression disguised as ethnic conflict.
We supported the Europeans when they asked for United Nations peacekeepers, from Britain, France, Sweden and even Bangladesh.
That also failed.
Only when the U.S. took the lead and applied military power to reinforce diplomacy did we halt the conflict.
And we did succeed in ending it with minimal expense and without losing a single soldier.
In Ukraine today, Russian-backed forces continue to reinforce and attack Ukrainian positions.
The Minsk II agreement that calls for a cease-fire, pullback of heavy weapons, and withdrawal of foreign forces hasn't been implemented.
Losses on both sides are heavy, far heavier than publicly acknowledged.
Russia is using its newest equipment — tanks, long range rockets, cluster munitions, drones, electronic warfare — to slowly grind away Ukrainian forces that lack modern equipment.
Russia, of course, still denies its troops are present:
This is "hybrid warfare," military aggression covered by the cloak of lies and propaganda.
But, actually, except perhaps for a few stubborn European diplomats, there is surprisingly little dispute as to the facts.
German Chancellor Angela Merkel insists there is no military solution — but, as in the Balkans, there will be no diplomatic solution until the military "door" is closed for Russian President Vladimir Putin.
And closing the door is actually simpler than many would have you believe.
According to Ukrainian sources, Putin ordered Gen. Valery Gerasimov, chief of the Russian general staff, to seize territory out to the provincial boundaries of Luhansk and Donetsk by Jan. 31.
Russia failed to secure these provinces in the face of stubborn and heroic Ukrainian resistance.
But as a result, Ukraine's forces are much weaker, while Russia continues to pour in tanks and artillery.
For diplomacy to work, the front must be stabilized.
Ukraine needs the means to defend itself: anti-armor, counterfire radar, drones, night vision capabilities and secure communications.
All this is readily available from the stocks of the United States, Poland and other allies.
It requires no U.S. soldiers in the fight and no U.S. air power.
It is not a proxy war against Russia; it is simple assistance to a fledgling democracy seeking the right to choose its own course.
The U.S. should take the lead now, as we did in the Balkans:
Tell Putin he'll get some eventual phased sanctions relief if he halts aggression, pulls back and obeys international norms of behavior.
The Minsk II agreement is a starting point, but Russia needs to recognize all Ukraine's borders, including Crimea.
If not, the Ukrainians will receive all the arms they need to stop his aggression.
This can all be couched in the normal diplomatic terms, and we can invite Germany to come along to deliver the message.
In the meantime, we need to accelerate the delivery of the minimal assistance we have already promised and encourage our allies to immediately deliver anti-armor and artillery ammunition.
Some will say this won't work because Putin will simply reinforce, but there are limits to Russian power, even on its borders.
After six trips to Ukraine, including meetings with the Ukrainian president and defense minister, I have come away impressed with Ukrainians' determination.
They will fight hard.
Meanwhile, Putin is still trying to disguise Russian aggression from his own populace.
Russian losses are increasingly difficult to conceal.
Others say Putin might retaliate elsewhere, with a wider war, or break off cooperation on the Iranian nuclear weapons talks.
But if Putin seeks a wider war, far better to find out now than when he has digested Ukraine and is on NATO's borders.
So far as his participation in the Iran talks are concerned, he knows that this is his most powerful leverage.
He's unlikely to throw it away.
As a senior officer who worked with Richard Holbrooke on the Dayton peace accords, and later as NATO commander for the peace implementation, I find all the arguments about Ukraine depressingly familiar.
What is new is America's reluctance to understand and fulfill its leading role as the guarantor of peace and security in Europe.
Since the end of World War II, the U.S. has remained deeply engaged in European security.
We recognized that our security depended on a free, democratic and peaceful Europe.
During the Cold War, we maintained 400,000 servicemembers there to deter the Soviet Union.
In the post-Cold War, we acted to bring peace to Bosnia and halt ethnic cleansing in Kosovo.
Today, our challenge is Russian aggression in Ukraine, contravening international law, threatening stability in Europe.
We cannot recreate American prosperity, ameliorate income inequality, "pivot" to Asia, or deal with international terrorism without stability and support from Europe.
Strategic patience will fail if we accept Russian aggression in Ukraine.
It is time for America to lead.
Retired General Wesley K. Clark, a former supreme commander of NATO, led alliance military forces in the Kosovo War. He is a senior fellow at the Burkle Center for International Relations at UCLA and author of Don't Wait for the Next War: A Strategy for American Growth and Global Leadership. Clark's Ukraine trips were paid for by the Potomac Foundation and the Open Society Institute.
Source: USA Today