BRUSSELS — President Obama started his day in Warsaw struggling to convince his friends in Central and Eastern Europe that the United States is being tough enough with Russia. He ended his day in Brussels, still struggling, but this time to persuade America’s core Western allies to stay tough with Russia.

The dizzying contrasts underscored the challenges Mr. Obama faces navigating the complicated waters of European politics as he tries to forge a unified stance against Russian aggression in Ukraine. On the defensive at home for a prisoner swap, he finds himself pressed overseas by some allies unsatisfied with his reassurances of resolve and others unimpressed with his arguments for action.

He arrived here on Wednesday to have dinner with the leaders of the Group of 7 powers who, at his urging, had excluded President Vladimir V. Putin of Russia as punishment for his annexation of Crimea. But Mr. Obama’s counterparts from Britain, France and Germany all ended up scheduling one-on-one meetings with Mr. Putin later on. President François Hollande of France even arranged to have dinner with Mr. Putin on Thursday just after having a separate dinner with Mr. Obama.

Not only were they unwilling to snub the Russian leader entirely, as Mr. Obama sought, they were also reluctant to go along with other efforts to isolate the Kremlin. Most notably, the French government repeated that it would go ahead with the $1.6 billion sale of powerful warships to Moscow along with plans to train 400 Russian sailors in France this month. And other European leaders were cautious about setting further red lines threatening additional sanctions against Russia.