A Ukrainian serviceman checks his machine-gun. (Anatolii Stepanov/AFP/Getty Images)

John McCain, a Republican from Arizona, is chairman of the Senate Armed Services Committee.

Last weekend, I traveled with Sens. John Barrasso (R-Wyo.) and Tom Cotton (R-Ark.) to eastern Ukraine to meet with the courageous men and women fighting there for their country’s freedom and future. I arrived on a solemn day as Ukrainian volunteers grieved the loss of two young comrades killed by Russian artillery the day before. They had lost another comrade a few days before that, and four more the previous week. Their message to me was clear: The cease-fire with Russia is fiction, and U.S. assistance is vital to deterring further Russian aggression.

Along the front lines, separatist forces backed by Russia violate the cease-fire every day with heavy artillery barrages and tank attacks. Gunbattles are a daily routine, and communities at the front bear the brunt of constant sniper fire and nightly skirmishes.

Yet while these low-level cease-fire violations have occurred regularly since the Minsk agreement was signed in February, Ukrainian battalion commanders said the number of Grad rocket strikes and incidents of intense artillery shelling are increasing. Their reports suggest that the separatists have moved their heavy weapons and equipment back to the front lines hoping to escalate the situation. So far, Ukrainian armed forces supported by volunteer battalions have been able to hold their ground, and they have done so largely without the support of Ukrainian artillery and tanks that have been pulled back from the front as stipulated by the Minsk agreement. How long can we expect these brave Ukrainians to abide by an agreement that Russia has clearly ignored?

It is time that the United States and our European allies recognize the failure of the Minsk agreement and respond with more than empty rhetoric. Ukraine’s leaders describe Russian President Vladimir Putin’s strategy as a game of “Pac-Man” — taking bite after bite out of Ukraine in small enough portions that it does not trigger a large-scale international response. But at this point it should be clear to all that Putin does not want a diplomatic solution to the conflict. He wants to dominate Ukraine, along with Russia’s other neighbors.

No one in the West wants a return to the Cold War. But we must recognize that we are confronting a Russian ruler who seeks exactly that. It is time for U.S. strategy to adjust to the reality of a revanchist Russia with a modernized military that is willing to use force not as a last resort, but as a primary tool to achieve its neo-imperial objectives. We must do more to deter Russia by increasing the military costs of its aggression, starting with the immediate provision of the defensive weapons and other assistance the Ukrainians desperately need.

President Obama has wrongly argued that providing Ukraine with the assistance and equipment it needs to defend itself would only provoke Russia. Putin needed no provocation to invade Ukraine and annex Crimea. Rather, it is the weakness of the collective U.S. and European response that provokes the very aggression we seek to avoid. Of course, there is no military solution in Ukraine, but there is a clear military dimension to achieving a political solution. If Ukrainians are given the assistance they need and the military cost is raised for the Russian forces that have invaded their country, Putin will be forced to determine how long he can sustain a war he tells his people is not happening.

I urge anyone who sees Ukraine’s fight against a more advanced Russian military as hopeless to travel to meet those fighting and dying to protect their homeland. These men and women have not backed down, and they will continue to fight for their country with or without the U.S. support they need and deserve.

During my trip, the Ukrainians never asked for the United States to send troops to do their fighting. Ukrainians only hope that the United States will once again open the arsenal of democracy that has allowed free people to defend themselves so many times before.

How we respond to Putin’s brazen aggression will have repercussions far beyond Ukraine. We face the reality of a challenge that many assumed was resigned to the history books: a strong, militarily capable state that is hostile to our interests and our values and seeks to overturn the rules-based international order that American leaders of both parties have sought to maintain since World War II. Among the core principles of that order is the conviction that might does not make right, that the strong should not be allowed to dominate the weak and that wars of aggression should be relegated to the bloody past.

Around the world, friend and foe alike are watching to see whether the United States will once again summon its power and influence to defend the international system that has kept the peace for decades. We must not fail this test.