“That the Russians would be happy burglarizing the emails of a major party to try to affect the outcome of our presidential election . . . is very serious and an unprecedented development,” former Maryland governor Martin O’Malley told Fox Business. Slate’s Franklin Foer called it “a strike against our civic infrastructure” that violates “a clear set of rules designed to limit foreign interference in our elections.”

Without context, that outrage is naive. Foreign governments have regularly sought to shape our politics. And the United States, in addition to overtly sponsoring regime change, has honed interference in other countries’ elections into something of an art form. Such interventions will always be appealing to their perpetrators because they can succeed, especially if they find willing accomplices in the targeted country.

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What is unprecedented in the United States, though, is Trump’s response. Never before has an American politician actively encouraged foreign intervention in a U.S. election — as Trump did with his invitation to Russia to hack into Hillary Clinton’s emails. It’s Trump, not Russia, who has violated established norms.

Great-power interference in American politics goes back to the infancy of the republic, when fears of French covert operations designed to drag the United States into a war with Britain led President John Adams to sign the Alien and Sedition Acts of 1798. As historian Timothy Naftali notes, before Pearl Harbor, Britain tried to turn U.S. public opinion against isolationists such as Charles Lindbergh and the original America First movement.

Perhaps the most sustained attempt by a foreign power to exert influence in American domestic politics took place during the Cold War. As Christopher Andrew recounts in “The Sword and the Shield,” “influence operations” — which spread disinformation — were staples of Soviet intelligence activities. Notable successes included Soviet dissemination of conspiracy theories about the assassination of President John F. Kennedy and smears about the private lives of officials such as J. Edgar Hoover.

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The KGB also sought to inflame U.S. racial tensions in the late 1960s and early 1970s. A 1971 operation fabricated racist pamphlets, attributed them to the Jewish Defense League and mailed them to militant African American organizations. After the assassination of Martin Luther King Jr., the KGB reprised its conspiracy-theory peddling to spread rumors that the government had connived in his death. Russian officials hoped that these efforts would diminish Americans’ trust in their government and sow confusion.

The United States also has a long and active history of interventions in other countries’ politics. We have toppled governments by supporting coups, fueling revolutions and sending in our troops. We have employed more subtle tactics, too, to influence the outcome of elections.

In 1948, U.S. policymakers feared that Soviet-backed communists would win power in Italy. In response, as John Lewis Gaddis discusses in “The Cold War: A New History,” the role of the newly created CIA was extended beyond intelligence-gathering to allow the agency to funnel money and organizational support to pro-U.S. parties. American assistance may have included forging documents to discredit the Communist Party.

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After Washington’s favored party won the Italian elections, such interventions became a staple of global-power politics. Political scientist Dov Levin estimates in International Studies Quarterly that Washington and Moscow intervened in a third country’s elections 117 times between 1946 and 2000. Sometimes, those interventions were overt, as when U.S. officials went out of their way to show favor to Chancellor Konrad Adenauer in West Germany’s 1953 elections. At other times, the interventions were kept secret, as with American support for Thai political parties in 1969.

The United States and Russia (or the Soviet Union) have meddled in elections because it has served their national interests and because the inherent risk has often paid off. Levin estimates that an overt intervention by a superpower yields a tilt toward its desired outcome equal to about 3 percent of the total vote. In a close election (such as West Germany’s in 1972 or Israel’s in 1992), that effect could easily be large enough to tip the balance.

Yet Levin’s research suggests that we also shouldn’t exaggerate the influence of interventions. Great powers can’t simply hand an election to their favored candidate. At best, they can swing support from a candidate they oppose to the most popular candidate they find acceptable.

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In other words, even if Moscow is using its resources to back Trump’s campaign, that doesn’t mean Trump is a Manchurian (Muscovite?) candidate bent on doing Vladi­mir Putin’s will. It could simply mean that the Kremlin thinks it could benefit more from a President Trump than from a Hillary Clinton administration.

More generally, an intervention can succeed only if someone in the targeted country cooperates with the outside power. American and Soviet interventions worked because the rival superpowers could find parties whose beliefs (or greed) proved simpatico. But the Soviets never successfully intervened in a U.S. election, because no major political figure wanted to be associated with the Soviet Union. Collaboration would have been unthinkable.

Until this past week, one might have thought a similar taboo would prevent candidates from capitalizing on foreign efforts to influence U.S. elections. Candidates might be tempted to give credence to leaked documents, especially if juicy revelations have the potential to undermine a rival. But you’d expect them to keep their distance. After all, the documents could easily be forged. And, more important, a foreign power deciding what gets released and when will always favor its own interests over the interests of the United States.

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The best strategy for a U.S. candidate, then, is to publicly denounce such leaks as unacceptable and dangerous, no matter which party they benefit. Yet the Trump campaign has embraced the tactical upside of the scandal, with the candidate himself even urging Russia on. “If Russia or any other country or person has Hillary Clinton’s 33,000 illegally deleted emails, perhaps they should share them with the FBI!” Trump tweeted. (He later said he was being sarcastic.)

Regardless of whether this document leak resulted from a foreign government’s actions, Trump’s response has once again weakened the foundation of American democracy.