But the Russian challenge is even broader than hybrid warfare. The Kremlin is attacking the rules that have guided relations among nations since World War II, rules that kept the peace among major European powers for 70 years. With their invasion of Ukraine in 2014, Mr. Putin trashed those rules, spurned international consensus, violated the treaties and principles that even previous Russian and Soviet leaders had respected — even in the breach.

Mr. Putin seems to want to return to the law of the jungle that characterized relations among nations for centuries before 1945, where powerful nations dominated and invaded less powerful nations, where nations established spheres of influence that oppressed neighbors, leading to war and suffering. That was how the Russian Empire and Soviet Union conducted international relations — dominate, control and absorb neighboring lands. A return to jungle rules threatens not just Ukraine and the United States, but global stability itself.

Until Russia withdraws from Ukraine — both Donbas and Crimea — and recognizes that Ukraine is an independent, sovereign nation, other nations cannot be secure. Until Russia recommits to a rules-based international order, Western nations are in jeopardy. Ukraine is the front line.