George Santayana famously said, “Those who forget the past are condemned to repeat it.” But when it comes to Russia and Ukraine, Western leaders can’t seem to decide which century’s lessons they should take to heart.

When Vladimir Putin’s forces invaded Crimea, a stunned Secretary of State John Kerry initially opined: “It’s a 19th-century act in the 21st century.” In other words, 21st-century rules of an interconnected world barred anything as atavistic as forceful seizure of European territory. Kerry was so stunned he could only suggest as punishment that Russia be kicked out of the club of G-8 industrial nations.

Initially, German Chancellor Angela Merkel seemed equally flummoxed, saying she thought “we had transcended” 20th-century conflicts over territory. Germans are now debating whether World War I or II holds the more relevant lessons for Crimea: Should Europe go slow on Putin lest it sleepwalk into war as it did in 1914? Or does it need a military response to Putin’s anschluss of the sort Europe failed to provide in 1938 when Hitler annexed Austria?

The problem with swallowing Santayana’s warning whole is that history never repeats itself to the letter. Any attempt to base current strategy on what happened 75 or 100 years ago is likely to end badly for all concerned.

Yes, Putin has reintroduced into Europe the pre-1945 concept of annexing territory by force — which has not been seen since the onset of the nuclear era. He can’t justify this practice by claiming he has a duty to protect Russian speakers outside Russia proper (who weren’t in any danger). “That’s like saying France has a duty to protect French speakers in Belgium or Switzerland,” says Francois Heisbourg, director of the International Institute for Strategic Studies.

Or, more ominously, like saying China has a right to seize Taiwan or islands whose possession is disputed with Japan or the Philippines.

Heisbourg says it’s time for Europe to abandon its illusion that hard power is passe, and to “secure our own strategic borders.” This does not mean, however, that it’s time to gear up for World War III. Putin isn’t Hitler, nor can an economically weak Russia afford a military clash with NATO neighbors.

But it does mean President Obama, Kerry, Merkel, and other European leaders need to abandon the fantasy that Moscow — or Beijing — would never use 20th-century military tools.

The idea is not to go to war with Russia, but to convince Putin that he faces serious consequences should he invade Ukraine proper — or try to annex bits of other small, ex-communist neighboring nations. So far, the signals from the West have been too weak to impress the Russian leader.

True, U.S. visa restrictions and asset freezes on a few of Putin’s oligarch cronies have unsettled Russia’s stock market and given foreign investors jitters. But European nations, heavily dependent on Russian gas, investment, and markets, have been far more timid. Yet Europe is in a good position to do far more.

In the short term, Putin can’t credibly use the threat of a gas cutoff to scare off stronger European action. “If this were December, Europe would be scared,” says Mikhail Korchemkin, executive director of East European Gas Analysis, a consulting firm in Malvern. “But it’s spring,” meaning gas demand is down. In addition, Europe has large reserve stocks.

Moreover, any threat to turn off the gas is self-defeating because Moscow’s budget depends on gas revenue. “Europe could survive for six to eight months,” says Korchemkin, “and Gazprom (Russia’s state-controlled gas company, whose profits fuel the Kremlin) could not.”

Furthermore, America’s shale-gas revolution — creating a switch from gas importer to soon-to-be exporter — has changed the global energy balance; this means new resources are available elsewhere for Europe.

So it is Western leaders who should brandish the energy weapon when they gather in Europe (while also pledging strong economic support for Ukraine, along with weapons for self-defense).

To make the point more bluntly, the Group of 7 (now minus Moscow) should declare that if Russian troops enter Ukraine proper, that will automatically trigger sanctions on badly needed foreign investment in Russia’s energy sector, similar to sanctions on Iran. Given Putin’s dependence on Gazprom profits, that should get his attention.

“It is very important to show Putin he cannot play the clock, that we have the staying power and he doesn’t,” says Heisbourg. That will require paying attention to the lessons of the 20th century while using 21st-century economic weapons that are as powerful as guns.