The 22 former leaders said they still feel bullied by Russia and claimed that Moscow continues to challenge their sovereignty.

They warned in a letter, carried on the web site of the Gazeta Wyborcza newspaper and to be delivered in Washington later Thursday, that U.S. credibility would be damaged if Washington abandons plans for a missile defense shield in response to Russian pressure, calling this the “thorniest” of current issues.

The administration of former President George W. Bush reached agreements last year to station interceptor missiles at a base in Poland and a linked radar base in the Czech Republic. Russia vehemently opposes the plan, and Obama is skeptical of it and is undertaking a thorough review.

“Abandoning the program entirely or involving Russia too deeply in it without consulting Poland or the Czech Republic can undermine the credibility of the United States across the whole region,” according to the letter signed by former leaders from countries once in the Soviet-controlled communist bloc but now NATO and EU members.

The signatories include former Presidents Lech Walesa and Aleksander Kwasniewski of Poland, Vaclav Havel of the Czech Republic, Emil Constantinescu of Romania and Vaira Vike-Freiberga of Latvia. They describe themselves as U.S. allies who remain deeply indebted to Washington for helping bring down the Iron Curtain.

“Had a ‘realist’ view prevailed in the early 1990s, we would not be in NATO today and the idea of a Europe whole, free, and at peace would be a distant dream,” the letter said.

The letter comes days after an Obama visit to Moscow, where he sought to reboot the tense relationship between the United States and Russia, including by putting a renewed focus on paring down nuclear stockpiles.

While missile defense came up in the talks, there was no progress and further discussions were put aside for later.

Russia has threatened to deploy missiles near Poland if the United States pushes ahead with the shield. Obama has attempted to reassure Moscow that the system is geared to tempering a ballistic missile threat from countries like Iran, a strong trading partner of Russia.

There is “nervousness in our capitals,” the authors wrote. “We want to ensure that too narrow an understanding of Western interests does not lead to the wrong concession to Russia.”

While they welcome Obama’s attempts to “reset” ties with Russia, they warned that Russia still acts as if it has final say in the region.

“Our hopes that relations with Russia would improve and that Moscow would finally fully accept our complete sovereignty and independence after joining NATO and the EU have not been fulfilled,” the letter says. “Instead, Russia is back as a revisionist power pursuing a 19th-century agenda with 21st-century tactics and methods.”

The authors cite economic warfare, including recent gas shortages in past years with Ukraine and others; politically motivated investments; bribery and media manipulation in order to advance its interests and to challenge the trans-Atlantic orientation of Central and Eastern Europe.

A security analyst, Bartosz Wisniewski, said the letter is more “alarmist and defensive in tone” than the reality would dictate, especially since Obama has tried to reassure the region that he would not sacrifice its interests in his rapprochement with Russia.

“But if you want to get something across in Washington you need to be vocal,” said Wisniewski, who works for a state-funded think tank, the Polish Institute of International Affairs.

He said it is notable what is absent from the 3,000-word letter. There is no mention of Ukraine or desires for further NATO enlargement and only the briefest mention of Georgia.

“It’s about consolidating what has already been gained in the last 20 years and taking care of the allies that you already have and not thinking about the others,” Wisniewski said. “It is a very realistic approach given the priorities of the Obama administration.”