Part 3 in a Series

Donald Trump was elected to president on a platform of politics not as usual, so it is fitting he inherits a world in flux. Post-World War II rules are dying, old alliances shifting and traditional roles shed. While Trump is a giant question mark on the world stage, NBC News' Chief Global Correspondent Bill Neely looks at major international challenges the president-elect faces upon inauguration on Jan. 20.
LONDON — Nobody is more likely to test the new administration than Russia's Vladimir Putin.
Putin has already overlapped with three other American presidents and his government's actions colored the U.S. campaign long before Trump's November 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 told NBC News.