WARSAW (AFP) - US President Barack Obama arrived in Poland Friday for a summit with leaders of ex-communist nations on the lessons of their decades of reform for the Arab Spring, with Belarus singled out as a transition gone wrong. Obama, who has said the track record of countries once cloaked behind the Iron Curtain could be instructive for Arab nations, was scheduled to join more than a dozen heads of state for a working dinner. Earlier, Poland's President Bronislaw Komorowski said the key question was: "How can we support democratic transformation in our region and beyond it, in the neighbourhood of the European Union both to the east and to the south?" Europe's communist regimes crumbled in 1989-1991, paving the way for democracy and free market reforms in most cases. Estonia's President Toomas Hendrik Ilves said such transitions need careful stewardship. "Changing policy alone is not enough to win a victory over authoritarian or totalitarian regimes and achieve democracy. We have seen this right here in Europe," Ilves was quoted as saying in a statement issued by his office. He singled out Belarus, run by authoritarian President Alexander Lukashenko since 1994, where a regime crackdown including Thursday's sentencing of opposition leaders has sparked resounding Western condemnation. Senior Obama adviser Ben Rhodes told reporters Belarus was "the most troublesome remaining country in Europe in terms of backsliding on democracy." Liz Sherwood-Randall, senior director for European affairs at the US National Security Council, underlined the role ex-communist states can play elsewhere. "One of the themes of our administration is that these countries that moved along towards democracy at the end of the Cold War have great experience to share with those countries that have not yet made that transition," she said. Ilves said "successful post-communist countries should feel obliged to help" North African and Middle East nations as they cast off "stagnated and authoritarian regimes". In 1989, Poland was the first country in the Soviet bloc to shed communism, in a bloodless transition negotiated between the regime and the Solidarity opposition, led by Lech Walesa. That sparked a domino effect with the entire bloc crumbling by the end of 1991. "Of course there are some differences between our case and the Arab Spring case. But the common thing is the excitement about developing democracy," Latvia's President Valdis Zatlers told AFP. He said outsiders should let Arab nations find their own path. "People are willing to assist in different ways. Nobody gave us democracy as a birthday present. We did it ourselves. And this is a very clear message that should be understood by all people who want positive change in that region," he explained. A swathe of ex-communist states have joined NATO since 1999 and entered the European Union since 2004. As it gears up for the EU's six-month rotating presidency in July, Poland is keen to focus on sharing the region's know-how, particularly with Egypt and Tunisia where Walesa visited recently. "Poland has taken the lead, to assist countries like Tunisia with their efforts to build new democratic institutions and processes," said Sherwood-Randall. Two weeks ago, Polish Foreign Minister Radoslaw Sikorski was also the first European minister to meet with Libya's rebel leaders at their Benghazi stronghold. The presidents of Albania, Bosnia-Hercegovina, Bulgaria, Croatia, the Czech Republic, Estonia, Hungary, Kosovo, Lithuania, Macedonia, Moldova, Montenegro, Slovenia, Slovakia and Ukraine were also at the summit. German President Christian Wulff - whose nation was split by the Iron Curtain until 1989 - and his Austrian and Italian counterparts also took part.