– As Russia consolidated its hold on Crimea, raising its flag over seized military bases and detaining ousted Ukrainian commanders on Sunday, President Obama and his European allies prepared to meet here in an effort to develop a strong, united response despite their diverging interests in dealing with the Kremlin.

After Russia’s invasion of Crimea and the lightning annexation of the peninsula by President Vladimir Putin last week, Obama’s decision to convene the leaders of several European countries, along with Canada and Japan, brought the nations — once again the Group of 7, without Russia — together for the first time since the crisis in Ukraine upended the stability and security of Europe.

National Security Adviser Susan Rice acknowledged that Obama’s weeklong trip, which includes a meeting with Pope Francis on Thursday and a stop in Saudi Arabia on Friday, would be overshadowed by Ukraine and the need to press for Western unity. Rice expressed confidence that the meeting here on Monday would “deepen” coordination.

But as the United States ratchets up economic sanctions against Russia, it may prove difficult for Obama to bring along his European allies, who are more economically intertwined with Russia and ended their own summit meeting on Friday with no detailed mention of more stringent sanctions.

A central question seems to be whether Western unity is more than a veneer of principled language and so-far mild sanctions, which, without any hint of a military response, has made the West seem powerless.

“It will expose the limitations within the European Union,” said Michael J. Geary, an assistant professor of modern Europe at Maastricht University in the Netherlands, noting that the bloc needs consensus among 28 member states that have disparate ways of dealing with Russia.

As the West has struggled to respond cohesively, Russia has moved assertively to establish control in Crimea. On Sunday, a base in Belbek was eerily quiet just 24 hours after it was seized in a dramatic incursion by Russian special-forces troops and two armored vehicles.

Russia solidifies hold

Although there were still scattered pockets of Ukrainian resistance across the peninsula on Sunday, it was clear that Russia was quickly locking down. Ukrainian military officials in Crimea said that bases continued to fall and that the Russian military had also detained a navy captain from a base near Sevastopol.

Even as Russian forces were storming two bases in Crimea on Saturday, the Kremlin agreed to allow monitors from the Organization for Security and Cooperation in Europe to start a six-month mission in Ukraine — but not in Crimea. The move seemed intended to ease fears that Russian forces would push into eastern or southern Ukraine.

But Putin’s recent record of first coy, then bold moves has put Obama and his European allies on guard. Caught flat-footed by the initial infiltration of Crimea, the United States seems increasingly alarmed about the 20,000 Russian troops that have massed on the border with eastern Ukraine.

The other members of the Group of 7 economic partnership hardly have interests identical with those of the United States, and in many ways they are divided even among themselves, complicating any effort to draw a firmer line with Moscow.

Obama’s sanctions, announced last week, were aimed at sowing pain among members of a Russian economic and political elite who owe their wealth and loyalties to Putin. But the sanctions were also targeted to minimize disruption to the global economy and to avoid further jeopardizing already meek Russian cooperation on such issues as the war in Syria, Iran’s nuclear program, the Middle East and North Korea.

For European countries, the risk of wider conflict with Russia is even graver. Britain hosts Russian billionaires and their money; Germany gets about one-third of its energy from Russia and sells it machinery and cars; France is in the process of delivering sophisticated attack ships to the Kremlin; and Italy depends on Russia for some 28 percent of its energy.

Crisis scenarios abound in a Europe alarmed by the Kremlin’s use of military muscle. Even normally dispassionate analysts have produced theories ranging from a thorough dismemberment of Moldova — which is already split by the pro-Kremlin enclave of Transnistria — to Russian intervention in eastern Ukraine.

A special role in the crisis has fallen to Germany, Europe’s biggest economy, and to Chancellor Angela Merkel, a Russian speaker who is familiar with Putin and in a stronger position at home than other leaders.

“Merkel certainly gets it,” said Charles Grant, director of the Center for European Reform, a nongovernmental group based in London.

Notoriously difficult to rattle in public, Merkel has employed unusually clear language in criticizing Putin, “which means she is really annoyed,” said Karl-Heinz Kamp of the NATO Defense College in Rome.

Germany seems to be trying, Kamp said, to differentiate between what might be called “Putin policy” and Russia’s broader national interests, which may be interpreted very differently by Putin’s eventual successor.

That long-term perspective alone illustrates how profoundly the events in Ukraine have shifted Western thinking.