In December of last year, two Russian intelligence officers met in the Washington, D.C., area with a potential source they hoped had valuable information regarding the inner workings of the Obama White House. At the meeting, the Russians probed for clues on what steps the Obama Administration—then in its final days—might take to punish Russia for its interference in the 2016 election.

According to a senior U.S. official, unbeknownst to the Russians, their source was not actually a White House mole but a U.S. counterintelligence asset. Of the options that the Obama Administration was considering, none trickled back to the Russians.

That particular Russian intelligence effort was thwarted, but a broader problem exists, according to the senior official. “There are more Russian operatives, declared and undeclared, in the United States now than at any other time in the past fifteen years,” the official told me. “They’re here in large numbers, actively trying to penetrate a whole host of sectors—government, industry, and academia.”

The F.B.I. is responsible for identifying and tracking foreign spies on American soil. But, the senior official said, the Bureau has “a math problem. It takes a lot of folks to run surveillance on one individual and make sure you never lose contact.” During the White House discussions that the pair of Russian officers tried and failed to learn more about, the F.B.I. director at the time, James Comey, pushed to expel as many Russian intelligence officers as possible. “This was a case-management issue for him,” the official said. At one point, a participant who took part in the discussions told me, one member of the national-security team also suggested sanctioning the Russian President, Vladimir Putin. Another proposed authorizing the National Security Agency to hack into the e-mail accounts of Russian officials in order to release embarrassing, possibly compromising, information.

On December 29th, the White House announced the expulsion of thirty-five Russian diplomats allegedly associated with Russia’s bid to “influence the election, erode faith in US democratic institutions, sow doubt about the integrity of our electoral process, and undermine confidence in the institutions of the US government.” The senior official wasn’t sure whether the two were among those expelled. Either way, “The number of folks who were sent home is a fraction of those who could have been sent home.”

Statecraft is built on expectations of proportionality, and deporting foreign spies is no exception. While throwing out thirty-five Russians relieved a strain on the F.B.I.’s counterintelligence agents, a proportionate response from Moscow—expelling thirty-five C.I.A. officers, for instance—would have created another, potentially more significant, problem in Russia. “We would be devastated there, and lose what little capability we have,” the senior official said. At one point during the Obama Administration’s deliberations, John Tefft, the U.S. Ambassador to Russia, sent a cable to Washington expressing these concerns, which were shared with the C.I.A. station chief in Moscow, according to the participant who took part in the discussions.

I asked Steve Hall, another former C.I.A. station chief in Moscow, about this apparent imbalance between American capabilities in Russia and Russia’s in the U.S. Hall said that he would not speak specifically about America’s intelligence-gathering strengths and weaknesses, but stated, “The Russians make it extremely difficult for American intelligence officers to operate in Russia, in a way that would be nearly impossible for the F.B.I. to do the same here.”

During the Cold War, C.I.A. officers in Russia carried out epic surveillance-detection runs, with American operatives travelling around Moscow for hours before making a dead drop to insure that they weren’t being tailed. Was the environment in Moscow now as restrictive as it was then? “It’s consistent to say that the resources and times and effort that it takes to run successful operations inside of Russia has not changed since Cold War times,” Hall said.

Inside the United States, in the meantime, Putin has overseen an apparent expansion of Russian intelligence operations. In March, 2001, the Bush Administration expelled fifty alleged Russian spies after the F.B.I. discovered that one of its own agents, Robert Hanssen, had been spying for Moscow. Nine years later, the F.B.I. arrested ten Russian “illegals”—intelligence officers who’d been living in the United States for years under deep cover. Russia’s cyber operations became increasingly sophisticated, too, launching paralyzing attacks against neighboring countries such as Estonia, Georgia, and Ukraine.

These wide-ranging new capabilities, coupled with Putin’s increased ambition, were displayed during the 2016 U.S. election, according to the director of National Intelligence’s unclassified January assessment. The Russian active-measures campaign was “multifaceted,” the report claimed, a “blend” of cyber activities and more overt disinformation campaigns. There was a strong human component, too. After Obama made his announcement about the December 29th expulsions, his Administration waited for a reciprocal response from Moscow. They didn’t know at the time that Donald Trump’s incoming national-security adviser, Michael Flynn, was calling the Russian Ambassador, Sergey Kislyak, to hint that the Trump Administration would review the sanctions when Trump took office. The next day, Putin said, “We will not expel anyone.”

Such good-will gestures now seem like relics of a distant era. Last month, after the House and Senate voted overwhelmingly in favor of a new round of sanctions against Russia, and the White House said that Trump would sign the bill, Putin, in retaliation, announced that his government would reduce the staff of the American Embassy and consulates in Russia by seven hundred and fifty-five employees. The reduction will primarily affect Russian nationals who work in the those facilities as translators, drivers, and support staff; it is not yet clear how many U.S. intelligence officers will be asked to leave Russia.

What’s evident is that tit-for-tat spy expulsions hurt Washington far more than they do Moscow. “In reality,” the senior officer said, “we have a lot more to lose than they do.”