Trump has intensified pressure on Ukraine to negotiate peace while adopting a softer stance on Russia. A report suggests Putin is open to a ceasefire, but only under conditions favouring Moscow, including excluding NATO forces and securing long-term security guarantees.

Trump's talks with the Russian president may endanger U.S. security interests if he "gives away his cards," Michael McFaul said.

Historians like to play a parlor game called periodization, in which they attempt to define an era, often by identifying it with the individual who most shaped the times: the Age of Jackson, the Age of Reagan. Usually, this exercise requires many decades of hindsight, but not so in the 21st century.

Advisers to the Russian leader have been surprised by the sudden change in tone from the White House in recent weeks.

In the fiery spat between Donald Trump and Ukrainian President Volodymyr Zelensky, it was a sentence that went largely unnoticed, but said quite a lot about the closeness the US leader thinks he has with Russia's Vladimir Putin.