Former Soviet leader Mikhail Gorbachev turned 90 on March 2, receiving greetings from the world leaders as well as the Kremlin.
Gorbachev is considered one of the greatest reformers of the 20th century.
After taking over the Soviet leadership in 1985, Gorbachev introduced his reform policies known as "glasnost" (openness) and "perestroika" (restructuring), which opened up the Soviet Union to the world, and ultimately led to the collapse of the communist regime and the end of the Cold War with the United States.
"Your commitment to freedom and your courage over the decades to make the tough, albeit necessary, decisions, have made the world a safer place," U.S. President Joe Biden wrote in a letter released by Gorbachev's staff.
The letter said that the agreed extension to the U.S.-Russian New START nuclear-arms treaty was proof that the two countries would continue Gorbechev's "legacy."
German Chancellor Angela Merkel congratulated Gorbachev, who was instrumental in the German reunification in 1990.
"I take your day of honor as an opportunity to thank you once more for your personal commitment for the peaceful overcoming of the Cold War and the completion of German unity," Merkel wrote in a letter to the former leader.
"Your important contribution to a reunification in freedom remains as unforgotten in Germany as your constant personal engagement for friendly relations between our two countries," Merkel said.
Russian President Vladimir Putin also congratulated Gorbachev, despite cool relations between the Kremlin and the former Soviet leader.
Gorbachev also owns a political foundation and co-owns the Kremlin-critical newspaper Novaya gazeta.
"You rightly belong to those bright, unconventional people, extraordinary statesmen of our time, who have had a significant impact on the course of national and international history," Putin wrote in a congratulatory letter to Gorbachev, published by the Kremlin.
Gorbachev's "great professional and life experience" still allowed him to "actively participate in popular social and educational work" as well as "international humanitarian projects," Putin wrote.
Many Russians also say he is responsible for the ensuing downfall of the Soviet Union in 1991. Putin himself has called the Soviet Union's collapse the "greatest geopolitical catastrophe" of the previous century.
A poll conducted by the All-Russia Opinion Research Center had 51 percent of respondents saying that he brought the nation more harm than good, while 32 percent said it was about equal, 7 percent viewed his action as mostly positive, and the rest were undecided. The nationwide poll of 1,600 was conducted on February 28 and had a margin of error of no more than 2.5 percentage points.
Pro-democratic forces, however, see him as a symbol of freedom, as he has criticized repression under Putin and warned against falling back into a dictatorship.