Moscow is on the march. Vladimir Putin’s Russia is the most destabilizing — and reckless — great power on the world stage. Following the collapse of the Soviet Union, Russia could have become a stable democracy at peace with its neighbors.

Instead, Mr. Putin is erecting a Great Russian empire. He has imposed a brutal police state at home. Journalists routinely are killed. Critics and dissidents are jailed. Media freedoms and opposition parties are under assault. A gangster elite runs the Kremlin, plundering the country’s vast wealth.

Russia has become a rogue state. Mr. Putin’s aim is to make Moscow the center of an anti-American, anti-Western axis. Russia has waged a genocidal war in Chechnya. It has de facto annexed the Georgian provinces of South Ossetia and Abkhazia. It has reduced Belarus to an economic vassal. It menaces the Baltic States. Moscow asserts a sphere of influence in Central Asia and the Caucasus. It has sold vital missile and nuclear technology to Iran’s mullahs. It has close ties with Hugo Chavez’s Venezuela.

Yet the Russian bear seeks an even bigger prize: Ukraine. This nation of 46 million, whose size is that of Germany and Britain combined, is of vital geopolitical importance — to both Russia and the West. Ukraine literally means borderland. Throughout the centuries, hostile neighbors — Russia, Poland, Lithuania — have sought to control Ukraine’s rich resources and minerals. Because of its geographic location, Ukraine’s fate has been to serve as a bridge between Asia and Europe; it straddles the civilizational fault line separating West and East. This is Ukraine’s curse and blessing.

Mr. Putin understands that his imperial ambitions ultimately can be achieved only if Ukraine is subjugated. Russia with Ukraine resembles America — a vast continental superpower. Without it, Russia is more like Canada — a large country mostly covered in snow.

Moreover, a democratic and prosperous Ukraine is a dagger aimed at the heart of the Putin regime. It will serve as a model for its northern Slavic cousins to imitate — a viable, attractive alternative to Mr. Putin’s barbarism. Hence, for Moscow, Ukraine must be smashed; its experiment in independence must be subverted.

Ukraine’s capital, Kiev, is a political battleground pitting pro-Russian forces against pro-Western nationalists. President Viktor Yanukovych is trying to roll back the clock to pre-Orange Revolution days. In 2004, backed by the Kremlin, he tried to steal the election, sparking street protests that culminated in the Orange Revolution. Earlier this year, he won elections — this time, fairly — on a platform of economic renewal and national reconciliation.

Mr. Yanukovych, however, has again proved the adage that you can’t teach an old dog new tricks. He is a Russophile thug who is slowly forging an authoritarian state. His government has centralized power, repealing amendments to the constitution — without public debate or any kind of vote — that substantially weaken parliament. Media censorship is on the rise. Journalists critical of the regime have disappeared mysteriously. In recent regional elections, opposition parties were harassed. Ballot tampering and voter fraud were rampant.

Mr. Yanukovych’s base is in the Russian-speaking parts of Ukraine — the Sovietized industrial east. His Party of Regions seeks to make Russian an official language; in fact, its website refuses to use Ukrainian. He has put joining NATO and the European Union on the back burner — bowing to Moscow’s demands. Slowly, but surely, he is splitting Kiev from the West. In short, he is Mr. Putin’s poodle.

The result is that Ukraine is a sovereign country in name only. Moscow funds Mr. Yanukovych’s Party of Regions and numerous Ukrainian think tanks and media outlets. The Kremlin has issued thousands of Russian passports in the Crimea, thereby creating Russian “citizens” who in the future may need “protection” from imaginary threats in Kiev — repeating the pattern established in Georgia. Also, the lease for Russia’s Black Sea Fleet, based in Sevastopol — set to expire in 2017 — was extended until 2042. Ukraine is being transformed into a Russian protectorate.

Mr. Putin despises Ukrainian nationalism. At a 2008 NATO meeting, the Russian strongman told then-President George W. Bush, “Ukraine is not a real country.” Rather, Mr. Putin said, it was a “gift” from Moscow, whose major territories formed part of czarist Russia. He publicly refers to Ukraine as “Little Russia.” His comments are not only insulting and disrespectful, but belligerent.

It is high time Washington takes notice. President Obama’s efforts to press the “reset button” in relations with the Kremlin have failed, emboldening Mr. Putin’s fascist regime. Ukraine’s descent into Putinism would be a tragedy of historical proportions. Contrary to Moscow’s propaganda, Ukraine is not a regional outpost of Russian civilization; rather, it is part of the European main — a long-suffering nation with a distinct cultural identity rooted in Western values, a separate language and unique Slavic heritage.

Ukraine is the eastern ramparts of the West. It is a strategic bulwark against Russian expansionism. It is not “Little Russia” but a nation in its own right. America cannot turn a blind eye. We must slap the bear down and tell Mr. Putin unequivocally to keep his greedy paws off Ukraine.