Editor's note: Sen. Kelly Ayotte, a Republican representing New Hampshire, is a member of the Senate Armed Services Committee. The opinions expressed in this commentary are hers.
(CNN) -- As Moscow escalates its invasion of eastern Ukraine and NATO convenes this week in Wales, the United States, NATO, and free nations around the world confront a pivotal moment of truth.
If the international community, led by the United States, fails to respond in a strong and unified manner to Moscow's blatant aggression in Ukraine, the ramifications will be both serious and far-reaching.
By now, it should be clear to all objective observers that Russian President Vladimir Putin is not impressed by starkly worded statements and declarations, and if that is the only outcome in Wales, it could represent a historic failure of the alliance at a time when NATO's foundational purpose has renewed relevance.
As NATO Secretary General Anders Fogh Rasmussen confirmed last week, Russian forces have "engaged in direct military operations inside Ukraine," including firing on Ukrainian forces from inside Ukraine itself. According to NATO, in addition to the Russian-backed separatists, there are now more than a thousand Russian military forces in Ukraine.
This latest escalation follows Russia's illegal annexation of the Crimea region of Ukraine, as well as months of instigating and perpetuating the crisis in eastern Ukraine. Even after the Russian-backed separatists apparently launched a missile that murdered 298 people on Malaysia Airlines Flight 17, Putin has continued to provide separatists with tanks, armored vehicles, artillery, rocket launchers and anti-aircraft weapons.
In short, Putin has brushed aside U.S. and European warnings, as well as initial sanctions, and has invaded eastern Ukraine.
The pre-eminent question now is what will the United States and NATO—and nations around the world that value democracy, freedom and the rule of law—do about it?
In May 2012, NATO governments declared that, "An independent, sovereign and stable Ukraine, firmly committed to democracy and the rule of law, is key to Euro-Atlantic security."
In his June 4 speech in Poland, President Obama said, "Our free nations will stand united so that further Russian provocations will only mean more isolation and costs for Russia."
As NATO declared at the 2010 Lisbon Conference, "Crises and conflicts beyond NATO's borders can pose a direct threat to the security of Alliance territory and populations." Russia's aggression in Ukraine certainly presents a threat to NATO.
The NATO Summit in Wales presents a critical opportunity to build the consensus necessary to back up these declarations with tangible and urgent action.
This action should include the toughest possible U.S. and European sanctions against Russia, the provision of much-needed and long-requested weapons to Ukraine, the strengthening of NATO's military posture in eastern Europe, the sharing of real-time intelligence with Kyiv, as well as robust economic assistance.
The United States and NATO do not need to send combat forces to Ukraine. The Ukrainian people have demonstrated a willingness to fight, and all they have asked for is our support.
As Europeans learned at a tremendous cost during World War II, weakness and delay in the face of invasion and aggression only invite more aggression. Thus far, the West's tepid response to Moscow's actions in Ukraine has only confirmed Putin's view that the United States and Europe lack the resolve to stand up to him -- and even worse, he views it as a green light for expanded aggression.
Continued weakness in the face of Putin's invasion risks leaving him with the dangerous impression that he can send his "little green men" to NATO member nations on Russia's periphery. Such a step by Moscow would trigger NATO's Article Five commitments and would require a NATO military response, as Gen. Philip Breedlove, NATO's Supreme Allied Commander Europe, has reiterated.
The United States and our NATO allies wish we had a well-intentioned partner in Moscow who follows international law and respects the sovereignty and territorial integrity of its neighbors. Despite NATO's best efforts to establish friendly and constructive relations with Russia since at least 1994, we clearly do not have a partner in Moscow.
Putin is a bully who wants to prevent the Ukrainian people and their legitimately elected government from choosing their own future based on his nostalgia for his days in the KGB, his desire to restore Russian dominance over its neighbors, and his fear that a democratic, independent and prosperous Ukraine might entice Russians to demand more accountability, democracy and prosperity at home.
The United States and our democratic allies in Europe and around the world are not and must not be neutral when a free people and their democratically elected government confront an unprovoked invasion of their sovereign territory.
Iranian Supreme Leader Ayatollah Ali Khamenei, Kim Jong Un in North Korea, the Taliban in Afghanistan, and Putin in Russia are watching. Do the United States and NATO possess the political will to back up their statements?
The answer to that question will have national security consequences for years to come in Europe and around the world.