When Kaja Kallas became Estonia’s prime minister she thought foreign policy was her weak spot. So she got to work. With recommendations from other leaders, Kallas drew up a study program. And she’s still at it.

She’s currently reading a history of Iran and has plowed through works by Margaret Thatcher, Henry Kissinger and a biography of Ukraine’s Volodymr Zelenskyy this year — as well as books about the Middle East, Israel-Palestine and Taiwan. Graham Allison’s "Destined for War," on the dangers of the U.S.-China rivalry, was suggested by the NATO secretary-general.

Foreign policy, she told Adam Grant on his Re: Thinking podcast last year, "is not my weakness anymore.”