In pictures: Obama-Merkel relationship

Barack Obama, ahead of his sixth and final trip to Germany as US president, described German Chancellor Angela Merkel as "probably... my closest international partner these last eight years".

Their relationship - captured in the famed 2015 photo above - has been described as a "rollercoaster romance".

But Mrs Merkel refused to let Mr Obama speak at the Brandenburg Gate when he visited Germany as a presidential hopeful back in 2008. He had to content himself with the Victory Column in the Tiergarten.

Relations in the early part of his presidency were overshadowed by differing views on the global financial crisis, a key issue at the 2009 G20 in London. An email to Hillary Clinton from one of her advisers, written five months later, said Mrs Merkel disliked "the atmospherics surrounding the Obama phenomenon".

And yet a warm relationship began to develop. Some commentators suggest the two - a lawyer and a chemist by training - found common ground in an analytical, pragmatic approach to policy.

Relations came under further pressure in April 2011 when Germany vetoed a plan for Nato intervention in Libya. But in June 2011, Mrs Merkel visited the White House and Mr Obama presented her with the US Medal of Freedom, lauding her as "a good friend and one of my closest global partners". The German media reacted cautiously, with one editorial questioning Mr Obama's "excessive hospitality".

Chancellor Merkel has said Mr Obama is "fun" to work with. She welcomed him on his first visit to Berlin as president in 2013. This time he did get to speak at the Brandenburg Gate.

But the relationship was tested again in October 2013, when leaked documents revealed that the US had spied on friendly foreign leaders, including monitoring the German Chancellor's personal mobile phone. At Mrs Merkel's next White House visit, the US media described the press conference as "frosty". Mrs Merkel faced problems with the translation equipment. Mr Obama said he was "pained" by the "strains in the relationship" caused by the revelations.

More recently, US-German relations have been warmer. The two have worked together on a wide range of issues, including an EU-US trade deal, Ukraine and the refugee crisis - on which Mr Obama has lauded Mrs Merkel for being "on the right side of history".

"For me, the future with the president is much more important than the past right now," Mrs Merkel declared in April 2016, according to CNN. Mr Obama said: "This is as important a relationship as I've had during the course of my presidency. Chancellor Merkel has been consistent. She has been steady. She is trustworthy. She has a really good sense of humour that she doesn't show all the time at press conferences."