Aerzen, Germany (CNN) President Barack Obama made a lengthy, personal plea for European unity Monday as the continent faces an uncertain future, casting a fractured union as catastrophic to global markets and stability.

Book-ending his landmark speech here eight years ago, which then-candidate Obama used to promote a reset in transatlantic ties, the President Monday said the same political forces he's attempted to combat in the United States threatened Europe. He said that in the years since he addressed Berliners in 2008, economic woes had worsened.

"In the vacuum, if we do not solve these problems, you start seeing those who would try to exploit these fears and frustrations and channel them in a destructive way," Obama said, describing "a creeping emergence of the kind of politics that the European project was founded to reject, an 'us' versus 'them' mentality that tries to blame our problems on the other."

Describing political trends, Obama cast what's happening in European countries in markedly similar terms to his analysis of his own country's political debate.

"You see increasing intolerance in our politics," Obama said. "And loud voices get the most attention."

Obama was visiting Europe as a series of global threats cause major strains in the bonds between the continent's nations. Terrorists have struck capitals in Belgium and France, leading to fears that further attacks are coming. Economic instability has led to slow growth and widespread unemployment. And aggression from Russia has spurred anxiety that traditional defense alliances are proving ineffective.

Those challenges are prompting some in the European Union to question its purpose, causing anxiety among U.S. officials who rely on the bloc in global negotiations. Before his stop in Germany, Obama made a stop in London to actively urge residents there to reject an exit from the EU.

In Hannover, where he's met for talks with German Chancellor Angela Merkel and toured an industrial trade fair, Obama broadened his message, saying that countries within Europe are better off together than apart.

"This is a defining moment. What happens here has consequences for people around the globe," Obama said. "I've come here today, to the heart of Europe, to say the United States, and the entire world, needs a strong and prosperous and democratic and united Europe."

Obama later sat for talks with the leaders of Europe's largest countries to reiterate his calls for greater unity among their nations. In his speech, he said that European nations must commit more to going after ISIS, and to ramp up their defenses against attacks at home.

"This remains a difficult fight and none of us can solve this problem by ourselves. Even as European countries make contributions to ISIL, Europe and NATO can still do more," he said, using a different acronym for ISIS.

Later, urging European countries to increase their defense spending to meet NATO requirements, Obama chided leaders for becoming over reliant on the United States for their own security.

"I'll be honest, sometimes Europe has been complacent about its own defense," he said.

Obama's speech in Germany came eight years after his 2008 speech to a 200,000-strong crowd in Berlin that greeted him rapturously. But the Europe Obama addresses now is starkly different. Eight years ago, amid descriptions of old-versus-new Europe, many on the continent were wondering about their relevance. Today, Europe is questioning its very survival.

"It's a very different Europe, one that's facing truly existential crises, with the future of the EU hanging in the balance, Russia's resurgence and a threat from within from jihadis as well as an economic crisis," said Derek Chollet, a senior adviser for security and defense policy at the German Marshall Fund.

"It's a Europe that's wondering about its own future and is looking to the United States for help and support," Chollet said.

In that light, Obama's choice to visit both the United Kingdom and Germany reflects an effort to support Europe and its unity -- perhaps a much more weighty mission than the official agenda of discussions on ISIS, counterterrorism, trade and other challenges, analysts said.

A UK departure would be a blow to the EU and Obama made clear before he even arrived in London that the U.S. would prefer Britain to stay within the Union. As the closest U.S. ally, the UK inside the EU can influence debates on economic and foreign policy and has added value to Washington in that position.

"The U.S. has a stake in the Brexit debate and vote because we want our No. 1 ally at the table, in the club," said Evelyn Farkas, a former deputy assistant secretary of defense who is now a senior fellow at the Atlantic Council.

Germany, said Karen Donfried, president of the German Marshall Fund, is the most consequential country in the union, while Merkel, with whom Obama is very close, is weaker than she's ever been politically.

"If President Obama wanted to do something to support the European project, which every U.S. administration has supported since the end of World War II, there are no more critical stops that he could make than in Britain and Germany," Donfried said.

Even Obama's trade-focused events in Germany have a subtext that's related to the debate over Europe's future, says Daniella Schwarzer, director of the Europe program at the German Marshall Fund's Berlin office.

"There's a very politicized debate about TTIP going on right now," Schwarzer said, referring to Obama's proposed transatlantic trade deals. While Germany is traditionally very supportive of trade agreements, "at this time we have a very strong and politicized debate that is as much about trade as it is about the legitimacy of the European Union."