Nato's leaders will gather this week to try to yoke together their fractious alliance after more than a decade of war in Afghanistan. But outside the convention centre, on the streets of Chicago, another coalition faces a major test of strength.
The Occupy movement will descend on Barack Obama's hometown intent on reasserting itself as a globally-heard voice for change. After months of setbacks and evictions - first from their spiritual home in Manhattan's Zuccotti Park, then St Paul's Cathedral in London and McPherson Square at the foot of the White House - Chicago offers a massive stage, lit by the cameras of the world's media.
"The media loves the story about Occupy dwindling but the truth is we're stronger than ever," said Rachael Perrotta, an organiser with Occupy Chicago. "These protests will only make us more cohesive and more powerful."
Thousands of protesters from around the US and the world are expected to take part, giving the Occupiers a chance to prove their relevance but also raising the spectre of widespread violence.
Fears of a repeat of the "Battle of Seattle" - the days of street fighting that marred the World Trade Organisation's 1999 gathering - are thought to be behind the White House's abrupt decision in March to separate the G8 summit from the Nato conference and move it away from Chicago.
Instead, the leaders of the world's largest economies will gather at Camp David, the presidential retreat hidden deep in the woods of Maryland behind miles of security fences and watch towers.
The closest protesters will be able to get will be the small towns of Frederick and Thurmont, where token demonstrations are expected to take place on Friday and Saturday before all eyes turn to Chicago.
When asked about security concerns for the Nato summit, Mr Obama casually replied: "We know how to deal with a crowd."
While it is true that Chicago has a history of dealing with protesters, not all its efforts have passed off peacefully. In 1968, angry crowds laid siege to the Democratic National Convention only to be met by widespread violence from police. Earlier this year, the city paid out $6.2 million (£4 million) in settlements after its officers were deemed to have made "mass arrests without justification" during a protest against the Iraq war.
At the helm of the city's response this week will be Mayor Rahm Emanuel, a former chief of staff in Mr Obama's White House. The Chicago Democrat - known for his profanity and short temper - is taking no chances and has already granted himself new powers to install CCTV, enlist federal officers in local enforcement, and sweep protesters out of public parks.
The officers of the Chicago Police Department have been fitted out with $1 million in new riot gear, including two "sound cannons" which can be used to emit painfully high frequency noise against crowds.
Chief Garry McCarthy insists that his department will focus on "extracting" trouble makers rather than trying to take on the demonstrators en masse. "If you treat people as individuals, they're individuals," he said. "If you treat them as a mob, they become a mob."
The Occupiers have less confidence in police restraint. "If you see violence it will be coming from the Chicago Police Department," said Mike McGuire, one of the activists heading to Camp David.
But while Occupy Chicago and its allies have committed to non-violence, their plans are seen as deeply provocative by the man they call "Mayor One Per Cent" and his police and already a dozen people have been arrested.
On Friday, protesters will gather at Daley Plaza, one of the city's main squares, where condemnation of Nato will be led by Tom Morello, the former front man of the polemical metal band Rage Against the Machine. Clashes are also expected on Saturday, as local demonstrators flood into Mr Emanuel's exclusive Lakeview neighbourhood to protest the closure of mental health facilities. His home address has already been widely circulated online.
In an echo of the 1968 protests against the Vietnam War, veterans from Afghanistan and Iraq will try to formally return their campaign medals to a US general on Sunday. If no officers will come meet them, the group of former soldiers intend to hurl their medals over the security fence lining the McCormick Place, the city convention centre.
Whether the summit passes peacefully remains to be seen. What we know is that the protesters will not let it pass quietly.
"We will be in the streets in such numbers and with such volume that they will hear us not only in McCormick Place but also at Camp David and around the world," Ms Perrotta said.