When the newly re-elected Barack Obama spoke at Berlin's Brandenburg Gate last June, he invoked the call of history: with the ghosts of John F Kennedy and Ronald Reagan hovering in the background, the newly-elected president urged his German audience to see itself as "part of something bigger".
"We are also citizens of the world. And our fates and fortunes are linked like never before," he said. "I say all this here, in the heart of Europe, because our shared past shows that none of these challenges can be met unless we see ourselves as part of something bigger than our own experience."
The message was clear: nearly 70 years after the end of the last World War the time has come for Europe's largest economy to shake off the understandable inhibitions that sprung from its role in that conflict and play a more constructive role in running the world.
On the face of it, Mr Obama and Mrs Merkel ought to understand each other well enough. Both are highly deliberative, pragmatic politicians who instinctively like to move by consensus and in increments, and yet over the last few years neither has shown much sympathy for the other.
Germany's decision to stand with Russia and China in abstaining from backing the UN Security Council proposal to enforce a no-fly zone over Libya scarred the relationship early on, sowing the seeds of a distrust that has never really been overcome.
Although such a raw split was avoided over Syria, it escaped no one's notice that of all the major European powers, only Germany failed to sign a statement on the fringes of the G20 condemning the Assad regime over its use of chemical weapons, with Berlin belatedly adding its signature a day later.
Mrs Merkel's handling of the euro-crisis, and her refusal to do more to underwrite European debt, has been another source of tension, particularly last year when the White House feared renewed economic crisis in Europe could have derailed Mr Obama's re-election.
That exasperation deepened this spring when Mrs Merkel stuck to her demands that ordinary Cypriot depositors must take a haircut in return for a €10bn bail-out. She was unmoved by American warnings that the move would panic depositors in other vulnerable Eurozone countries, like Spain and Italy – which it duly did.
Even on trade, where Germany as an exporter nation has an apparent vested interest in strengthening transatlantic ties, Mrs Merkel has not always been totally supportive, sometimes favouring German exports to China over the need to rebalance the world economy following the global financial crisis.
For America's foreign policy establishment, this about more than differences of opinion, but the much more fundamental question of whether Germany is prepared fully support the liberal world order that China's rise and Russia's Cold War posturing now calls into question.
In the near-term, the signs are not good. As Karl-Theodor zu Guttenberg, a former German defense minister wrote recently, there remains a "deeply entrenched culture of reluctance" for international engagement among Germany's political classes, that any German chancellor cannot ignore.
It may be a paradox that Europe's leading exporter cannot see the need to play a role in enforcing global security, but as Lars Hansel, the head of Konrad Adenauer Foundation in Washington says, the German public simply doesn't make – or want to make – the connection.
"Germans do not draw the conclusion that we need to be more engaged globally," he said. "This is something that people do not really internalise."
Mark Fisher of the German Marshall Fund puts German reluctance another way. Given their history, Germans are "terrified of being in the moral wrong" when it comes to acting in scenarios like Syria, he says, so they prefer taking no action at all.
In this, he argues, Germany has drawn the wrong lessons from history: the rise of Hitler and the embrace of Fascism taught Germany for evermore that it is "wrong to fight", when the real lesson should have been the opposite – that sometimes, "you have to fight against evil".
There is, of course, an inescapable irony that Mr Obama, of all US presidents, should be exhorting Mrs Merkel to confront her public over the dangers of isolationism.
Even three months ago, the call to Germany to do more to uphold the liberal post-War order was a stretch coming from Mr Obama whose own first term foreign-policy, particularly over Syria, had been so limited and constrained as at times to appear almost non-existent.
But over the last few weeks, as he tried and failed to argue the case for military intervention in Syria, Mr Obama has discovered just how difficult it can be to persuade the public to look beyond their immediate self-interest.
As Mr Obama correctly, but all too belatedly, observed the world order that has delivered seven decades of relative stability and rising prosperity must be underwritten by those countries that share the values of liberal democracy and free market capitalism on which it is founded.
"For nearly seven decades the United States has been the anchor of global security," Mr Obama said in his address to the nation.
"This has meant doing more than forging international agreements. It has meant enforcing them. The burdens of leadership are often heavy, but the world's a better place because we have borne them."
Unfortunately for Mr Obama the American public simply shrugged and said 'no'.
It is a rebuff that leaves Mr Obama weakened on many levels, including when it comes urging Mrs Merkel to make a case to the German public that, in word and deed, he has so glaringly failed to make himself.
Peter Foster is the Telegraph's US Editor based in Washington DC.