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While “there are some difficulties” still to discuss regarding missile defense, “we hope they will be resolved,” he said.

In joint remarks with the prime minister, Mr. Biden tried to convey America’s resolute support for the security of Poland.

Noting NATO’s collective security pledge, Mr. Biden said, “An attack on one is an attack on all. And this strategic assurance is absolute. Absolute, Mr. Prime Minister.”

Toby Gati, a senior adviser on Russia and Eurasia in the Clinton White House, said the outcome of the visit was favorable for the Poles.

“The Poles are going to do just fine under the new missile defense plans,” she said. “They were smart not to make a fuss and fight the Obama administration, but instead to try to figure out ways to enhance their security with the ‘new guys in charge.”

Zbigniew Brzezinski, national security adviser to President Carter, said Mr. Obama’s modified missile defense plan “is more realistic and doesn’t raise unnecessary issues with the Russians.”

While he faulted the Obama administration for “poor staff work” in timing the announcement — Sept. 17 was the 70th anniversary of the Soviet invasion of Poland during World War II — he said the “dispute is blowing over and Biden’s trip is very reassuring.”

Mrs. Gati said that the Kremlin may end up having “buyer’s remorse” over the U.S. policy shift.

“Yes, they got rid of the system Bush put forward, which they no doubt consider a political victory, but from the point of view of a U.S. ‘presence’ and a possible threat to Russia as the pieces of the system are integrated, this system may be of much greater concern,” Mrs. Gati said. “Unless, of course, we and the Russians do eventually cooperate on missile defense.”

As much as the visit was intended to look forward, Mr. Biden did make a brief detour to lay a wreath at the Warsaw Ghetto Monument, a stone edifice erected in 1948 in the memory of the Jews who rose up against the Nazis and those who were marched off to death.

In a light drizzle under an overcast sky, Mr. Biden quietly greeted two local rabbis, one who stood with a cane, in a corner of the cobblestone square where the monument stands. In a dark suit and no overcoat, he then walked with a Polish military officer to the center of the square.

Surrounded by 14 Polish soldiers, most holding raised rifles, Mr. Biden and his escort stood frozen as a bugle sounded. A snare drum snapped off a slow march, and the two took deliberate steps toward the 20-foot monument, which depicts resistance fighters gripping hand grenades.

Polish soldiers placed a wreath of crimson roses at the base. A sash across the wreath read, “In memory of the heroes of the Warsaw Ghetto uprising.” The bugler played taps as Mr. Biden held his hand over his heart. As he walked quietly away, a group of 10 Jewish schoolgirls sang an Israeli folk song. The vice president flashed a quick smile and waved at the schoolchildren before returning to the motorcade.

• Jon Ward contributed to this report in Washington.