Nobody is more likely to test the new administration than Russia's Vladimir Putin, according to NBC News.
The Russian president has already overlapped with three American presidents and his government's actions colored the U.S. campaign long before Trump's November election win.
U.S. intelligence officials believe with "a high level of confidence" that Putin became personally involved in the covert Russian campaign to interfere with the White House election, two senior officials have told NBC News.
Amid other moves, such as relocating nuclear weapons and venturing into neglected territories, Putin's Russia seems determined to challenge American leadership across the globe. Beneath that boldness, there is profound distrust between the two countries.