WASHINGTON  President Obama scrapped his predecessor’s proposed antiballistic missile shield in Eastern Europe on Thursday and ordered instead the development of a reconfigured system designed to shoot down short- and medium-range Iranian missiles.

In one of the biggest national security reversals of his young presidency, Mr. Obama canceled former President George W. Bush’s plans to station a radar facility in the Czech Republic and 10 ground-based interceptors in Poland. Instead, he plans to deploy smaller SM-3 interceptors by 2011, first aboard ships and later in Europe, possibly even in Poland or the Czech Republic.

Mr. Obama said that the new system “will provide stronger, smarter and swifter defenses of American forces and America’s allies” to meet a changing threat from Iran. Administration officials cited what they called accumulating evidence that Iran had made more progress than anticipated in building short- and medium-range missiles that could threaten Israel and Europe than it had in developing the intercontinental missiles that the Bush system was more suited to counter.

But the decision churned domestic and international politics as Republican critics at home accused Mr. Obama of betraying allies and caving in to Russian pressure, while officials in Eastern Europe expressed discomfort and confusion at the dramatic shift. President Dmitri A. Medvedev of Russia, who is to meet with Mr. Obama in New York next week, reacted cautiously as Moscow tried to determine whether the new system was less threatening to its own security.