Barack Obama will Wednesday propose major cuts in US and Russian nuclear stocks, making a pitch for his own place in history in an evocative open-air speech during his first visit as president to Berlin.
Almost 50 years to the day since John F. Kennedy declared "Ich bin ein Berliner" and 26 years since Ronald Reagan exhorted "Tear down this wall!" Obama will unveil plans for a one-third reduction in Cold War nuclear arsenals.
He also met German Chancellor Angela Merkel, with whom he usually has respectful relations, but who is pointedly demanding details on the exact extent of US spy agency surveillance programmes.
The US president was set to use his speech at Brandenburg Gate to propose cutting US and Russian strategic nuclear warheads to around 1,000 each, and also seek cuts in tactical nuclear arms stocks in Europe.
"We will seek to negotiate these reductions with Russia to continue to move beyond Cold War nuclear postures," a senior US official said.
It remains unclear whether Russian President Vladimir Putin, with whom Obama had a frosty meeting at the G8 summit in Northern Ireland on Monday, will agree to such substantial weapons cuts.
Russia has previously demanded changes to the US missile defence system before agreeing to return to the nuclear agenda.
Obama will also commit to attending a nuclear security summit in The Hague next year, and to hosting his own version in 2016 in the last year of his presidency.
Obama inaugurated the first such summit, designed to ensure unsecured nuclear stocks do not fall into the hands of terrorists, in Washington in 2010 and went to a follow-up meeting in Seoul two years later.
Wednesday's announcement is intended to ensure that his nuclear counter-proliferation agenda remains at the centre of his foreign policy legacy, following the conclusion of a Strategic Nuclear Arms Reduction Treaty with Russia during his first term.
The US has around 20 nuclear warheads still stationed in Germany, down from about 200 when the Berlin Wall fell in 1989.
Obama remains popular in Germany -- a poll this week for Die Zeit newspaper showed that 60 percent of Germans were satisfied with his leadership.
And 42 percent said he was more successful than Merkel, Germany's most popular post-war leader who is standing for a third term in September elections, versus 34 percent who thought the chancellor had achieved more.
But he will struggle to meet the expectations he spun for himself as a presidential candidate, in a speech to a crowd of 200,000 in Berlin in 2008 that made him a political star in Europe.
Since that call for a joint US-European bid to "remake the world" by battling terrorism, global warming, Middle East violence and poverty, Obama has learned the power of the status quo at home and abroad to thwart change.
But frustration will not temper his rhetoric, according to US deputy national security advisor Ben Rhodes.
"Any time a US president speaks in Berlin, it's a powerful backdrop to our post-war history," said Rhodes.
"This is a place where US presidents have gone to talk about the role of the free world.
"With that historical backdrop... sometimes it's easy to think that history is behind us, essentially. The Wall is down. There's not a threat of global nuclear war. The threats that we do face are far more distant.
"The overarching point that he's going to make is the exact same level of citizen and national activism that was characterised in the Kennedy speech and in the Cold War needs to be applied to the challenges we face now."
In his meeting with Merkel, Obama is under intense pressure to explain the scope of US National Security Agency spying programmes which hoover up data from phone records and the Internet in the United States and abroad.
The programmes, which have special resonance in a nation where snooping operations by the communist Stasi secret police are a painful memory, have triggered alarm in Berlin.
"I will call for more transparency," Merkel, who grew up in the communist East, said in an interview Monday, adding that Germans wanted to know if their online habits were being spied on by the NSA.
"We have to be clear -- what is being used, what is not being used," she said.
Obama, who arrived in Berlin on Tuesday from the G8 summit, has said he welcomes public debate on the trade-offs inherent between protecting privacy and citizens from the threat of terrorism.
His speech comes just ahead of the 50th anniversary of Kennedy's "Ich bin ein Berliner" address, two years after the erection of the Berlin Wall.
Brandenburg Gate itself was the backdrop for another climactic moment in Cold War history, when Reagan famously beseeched then Soviet leader Mikhail Gorbachev to "tear down this wall" in 1987.
Germany's troubled history was also the theme for First Lady Michelle Obama, who was joined by her two daughters on a tour that took in the Holocaust memorial and a stretch of the Berlin Wall.