No longer are diplomatic power struggles between nations played out in intense negotiations and high stakes summits. Instead, it's all in the handshake.

Donald Trump had left Shinzo Abe, the Japanese prime minister, reeling after a 19 second handshake last week in the White House.

Before that, the US president had grasped Theresa May's hand in an act that was splashed across the front pages as a sign of the "special relationship".

So when Justin Trudeau, the Canadian prime minister, became the third world leader to visit the White House on Monday, all eyes were on the all-important shaking of the hands as they met outside.