Russian President Vladimir Putin’s decision to mass nearly 100,000 Russian troops on the Ukrainian border has raised tensions in Europe to levels not seen since early in the Cold War. To forestall this attack, the G-7 nations have issued a stern warning that “Russia should be in no doubt that further military aggression against Ukraine would have massive consequences and severe cost in response.”

The allies have threatened a “punishing set of financial, technological, and military sanctions” that would come into effect immediately after a Russian invasion. Additionally, Senate Democrats have introduced the “Defending Ukrainian Sovereignty Act,” which would impose “crippling sanctions on the Russian banking sector and senior military and government officials.” Putin has responded to these threats by mobilizing additional Russian army units in the Crimea as well as in Belarus, which has alarmed NATO members bordering that country.

Putin’s objectives remain unclear. Is his troop deployment a way to pressure NATO into negotiating a new security arrangement for Europe, or even withdrawing NATO forces from Eastern Europe? Or does he believe he can present the West with a fait accompli by invading and installing a regime in Kyiv favorable to the Kremlin before the West can respond?

We do know one thing, however: An outright invasion promises huge consequences, a lesson Soviet leader Joseph Stalin learned in 1950 when he approved Kim Il Sun’s plan to invade South Korea. Kim had assured Stalin that the North Korean army would quickly capture Seoul and unite the country in only a few weeks, well before the United States could mount a response. Instead President Harry S. Truman immediately ordered units from the Eighth Army stationed in Japan to the Korean Peninsula as well as a massive bombing campaign to slow the North Korean advance. The invasion galvanized the West and transformed U.S. national security strategy, resulting in a massive mobilization, a decades-long nuclear arms race and a permanent militarization of the Cold War conflict.

Though the Cold War had begun several years before the North Korean invasion, American strategic planners considered military force a deterrent and saw the true field of battle as taking place in the realm of political and economic warfare. This was true even of NATO. When it was created in 1949, American policymakers believed that a public U.S. commitment to defend Europe would alone be sufficient to prevent a Soviet attack. The United States possessed a monopoly on the atomic bomb, and even more important, a vast economy that had repeatedly demonstrated its ability to outproduce its wartime rivals. As George Kennan argued in 1948, “the events of the past two wars have demonstrated that unless a European aggressor can be sure of dealing a decisive blow to the North American military-industrial potential in the initial phase of his effort to dominate the European continent, he can never be sure of final victory.”