Barack Obama's penchant for diplomacy and "soft power" is being tested by the burgeoning crisis in Ukraine.
The US president is seeking to punish Russia for annexing Crimea while trying to prevent a larger conflict should Moscow continue encroaching on its neighbours.
Lengthy phone calls between Obama and Vladimir Putin failed to persuade the Russian president to back off, despite Obama's efforts to improve relations.
Secretary of State John Kerry's last-minute marathon talks with his Russian counterpart also got the US no further.
The White House's initial round of sanctions that targeted Russian government officials were mocked in Moscow.
Washington moved on Thursday to expand the number of Russian officials being sanctioned - even as Obama insisted that there was a less confrontational option.
"Diplomacy between the United States and Russia continues," Obama said.
In Moscow, Russia's parliament voted to admit Crimea and the city of Sevastopol into the Russian federation and yesterday Putin signed the documents making it formal.
The treaty creates two new Russian administrative regions: Crimea and the port city of Sevastopol where Russia's Black Sea fleet is based.
Critics, as well as some foreign policy analysts, say Obama's initial response to Russia needed to be more muscular.
It required more than the nuanced approach the president has often preferred in international affairs since he took office in 2009 vowing to wind down a decade of war.
Barry Pavel, a former National Security Council staffer for both President George W. Bush and Obama, said: "My concern with the Obama approach is that it's very incremental, very measured.
"He's not been good at wielding negative tools. He's better at wielding positive tools."
Pavel, now vice-president of the Atlantic Council, a foreign policy think tank, pointed to the three-year-old conflict in Syria, where Obama has been reluctant to intervene.
Obama has insisted that Syrian President Bashar al-Assad must go. But Pavel said: "He never gave Assad a single negative incentive, never took a single negative action."
Similarly, the initial sanctions against Russia, Pavel said, were so limited that he feared Putin must be thinking: "Wow, they [the US] really are as wimpy as I thought they were."
Pavel added: "I would have taken sanctions and cranked them up to the point where they immediately cause Putin and his core accomplices to feel enormous economic and other pain."
Russia has also created a headache for German Chancellor Angela Merkel.
Despite Germany's heavy dependence on Russian oil and gas, extensive business interests in the country and long-held view of Moscow as a "strategic partner", Merkel struck quickly.
At a hastily called European Union summit in Brussels, Merkel was among the most vocal proponent of a tough line, sources said.
Yesterday the EU welcomed Ukraine into the Western fold, signing the political provisions of a landmark accord. The EU was offering Ukraine its "steadfast support," EU president Hermann Van Rompuy said, promising help to get the country's struggling economy back on track.
European Union leaders agreed sanctions against 12 more Russians with travel bans and asset freezes, bringing their list to 33. Among them was Russian Deputy Premier Dmitry Rogozin. Michael Geary, a European studies fellow at the Wilson Centre, a non-partisan global policy institution, said the White House's decision to sanction influential Russians in Putin's inner circle suggested a marked ratcheting-up in Obama's willingness to take a "proactive and indeed forceful line".
He said of Obama: "You can see an interesting contrast between mentioning red lines with Syria and the piecemeal approach he's adopting now with Russia."
Obama, who campaigned against Bush's launching of the war in Iraq a decade ago, has been criticised for taking too long to make up his mind on foreign policy threats, including the Syrian civil war. But some analysts say his deliberations may be more appropriate to the moment.
Andrew Weiss, vice-president at the Carnegie Endowment for International Peace, where he oversees research on Russia, said: "He's got to be careful and determined and not over-promise that he can put Humpty Dumpty back together again. There's a sense that we're hostage to provocations, and small-scale actors could trigger a further worsening of this crisis. And that's a place no president wants to be."
Obama's biggest hurdle was not of his making, Weiss said. Given the amount of trade Europe does with Russia, the continent had been reluctant to embrace sanctions against Russia that would bite too deeply, he said.
"The Europeans have US$400 billion or more at stake," Weiss said. "These are not nickel-and-dime issues. They go to the heart of their economic prosperity."
That made it important for the US to proceed carefully so it did not expose differences and a lack of unity, Weiss said.
Obama leaves this weekend for a trip to Europe likely to be focused on Ukraine. He has invited the leaders of the G-7 nations - formerly the G-8 before Russia was cast out - to meet.
Republican Senator John McCain, who said the administration's stance on Syria emboldened Putin, has called on Obama to send military aid to the Ukraine, as Kiev requested.
But Obama has ruled out military intervention in Ukraine. He said that engaging Russia militarily "would not be appropriate and wouldn't be good for Ukraine, either".
His remarks track public sentiment. A recent Pew Research Centre poll found that Americans were opposed to US involvement in Ukraine by nearly 2-to-1.
The poll also found that more people disapprove (44 per cent) than approve (30 per cent) of the way the administration is handling the situation.
William Pomeranz, deputy director of the Wilson Centre's Kennan Institute for Advanced Russian Studies in Washington, said Obama was open to criticism "mainly because there's so little he can do in Crimea on the ground that his response invariably looks not aggressive enough to Republicans".
He added: "They're moving in increments in response to what Russia is doing, but Obama also has the responsibility of making sure that this doesn't escalate into a bigger war. And no other politician has that responsibility to consider when they're making recommendations."
Additional reporting by ReutersTopics: Focus