Washington: For the first seven years of his presidency, Barack Obama could rely on the United Kingdom to back him up at nearly every turn.

Barack Obama
Barack Obama

Obama enters his final months in office with that sense of certainty shattered by the UK's decision to leave the European Union and the resignation of Prime Minister David Cameron, one of his closest collaborators on the global stage.

Britain's withdrawal from the union could take years.

White House officials say they expect no immediate changes to the myriad areas where the two countries are working together, including the British military's involvement in the US-led coalition fighting the Islamic State group.

Still, there's far less assurance that Britain and other European countries will reflexively take the US side as new and evolving challenges pop up.