Since then, the Trump White House has tipped the world order upside down with punitive trade tariffs and an unpredictable approach to countries including Ukraine, Russia, Greenland, Canada, Iran and Venezuela. This year, the conference - which begins on Friday - once again looks set to be decisive.
US Secretary of State and National Security Adviser Marco Rubio leads the US delegation, while more than 50 other world leaders have been invited. It comes as the security of Europe looks increasingly precarious and NATO allies are on edge.
The latest US National Security Strategy called on Europe to "stand on its own feet" and take "primary responsibility for its own defence," adding to fears that the US is largely unwilling to underpin Europe's defence.
The BBC's diplomatic correspondent James Landale takes us through the backdrop of the conference and the current state of the transatlantic alliance.
