In a press conference at the end of last week, U.S. President Joe Biden said he was “convinced” that Russian President Vladimir Putin had decided to invade Ukraine. Asked why, he said simply: “We have a significant intelligence capability.”

Understanding the intentions of a foreign autocratic leader, particularly one shielded from the outside world and reliant on a small group of trusted advisors, is the Holy Grail for any intelligence service. America’s spies, and their British colleagues, appear to have succeeded in that quest. We in the public are unlikely to know how until the relevant documents are declassified decades from now. But history can offer some hints about how Biden knows what he knows and why he has chosen to disclose some of this information publicly.

Cold War archives show that accurate warnings about an adversary’s intentions and capabilities were seldom, if ever, the result of a single kind of intelligence. Rather, they were invariably achieved through combinations of intelligence from human and technical sources. Today, open-source intelligence is also playing an increasingly important role. The specific mix of intelligence sources can influence what information a government publicly shares. As demonstrated during the 1962 Cuban Missile Crisis, presidents can best deploy intelligence in their diplomacy when the risk of burning sources is low.

The Human Factor

Human intelligence provides unique insights into what a foreign leader is thinking. This is especially the case with Putin, who, given his KGB background, is acutely aware of foreign intelligence capabilities and would resist risking interception by putting his intentions into writing before the last possible moment. Thus, even in an era of pervasive data, a human agent (or, more colloquially, a spy) with access to a foreign leader’s whispered secrets can still give unique insights into their mindset and motivations.

During the Cold War, it does not appear that any Western intelligence agency managed to recruit a spy with access to the innermost decision-making inside the Kremlin. The Soviet Union and its Eastern Bloc satellite states were, at key points during the conflict, graveyards for Western spy services. Ubiquitous surveillance in countries behind the Iron Curtain, severe restrictions on movements of Western intelligence officers working under diplomatic cover there (“your papers, please”), frequent intimidation (pictures rearranged in apartments, letting you know you’ve had a visitor), and physical harassment hamstrung their abilities to recruit and meet agents. Under relentless pressure in Moscow, Western intelligence officers burned out even after relatively short periods of service. In the later Cold War, the CIA would invent ingenious and elaborate disguises for its officers just to get out of U.S. embassies behind the Iron Curtain to meet sources.