WASHINGTON - President Barack Obama opens a six-day trip to Europe and Saudi Arabia on Monday seeking to reassure US allies amid the worst East-West crisis in years and to defend his nuclear diplomacy with Iran.
A visit to the third Nuclear Security Summit in The Hague -- which grew out of Obama's initiative to prevent the spread of weapons of mass destruction and radioactive material -- has been planned for months.
But it has been transformed into the most important trip to Europe by a US president in years because of the sudden crisis over Russia's annexation of Crimea.
Obama has called a G7 summit in The Hague to deepen his effort to isolate Russia and present it with "costs" over its incursion into the Ukrainian region.
Shockwaves from Crimea's return to Russia will overshadow The Hague summit and provide the subtext to Obama's onward trip to Brussels to meet EU and NATO leaders.
The trip comes as Western leaders rethink their relationship with Russia, following a post-Cold War period in which they sought to usher Moscow into the broader international community.
"I think for Mr Putin, the only thing that has surprised him these past three weeks from Washington was just how weak our response has been," said Andrew Kuchins, of the Center for Strategic and International Studies.
Washington has slapped visa bans and personal financial sanctions on some top officials in President Vladimir Putin's inner circle, but has yet to aim painful blows at the heart of Russia's economy.
One of Obama's core messages for the trip will be that Washington will stand by security guarantees for its NATO partners, including post-Soviet states who joined the alliance.
Obama will also send a message of solidarity to US allies when he sits down with South Korean President Park Geun-Hye and Japanese Prime Minister Shinzo Abe in The Hague.
Washington is keen that historical disputes between the two allies do not disrupt its Asian alliance network at a time of rising tensions between East Asian powers and China.
"If there's a common theme to this trip, it's the fundamental strength and importance of our alliances and partnerships," said National Security Adviser Susan Rice.
"The strategic importance of this effort really can't be overstated. From Europe to Asia to the Middle East, our ability to lead strong coalitions is essential to making progress," she said.
The president will also sit down with Chinese President Xi Jinping in The Hague in their first meeting this year.