Checkpoint Charlie was the best-known Berlin Wall crossing point between East Berlin and West Berlin during the Cold War, as named by the Western Allies. After the reunification of Germany, the American guard house at Checkpoint Charlie became a tourist attraction. Wikipedia
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In the bad old days of communism, Checkpoint Charlie was a crossing point between West Berlin and Soviet occupied East Berlin. It was one of the few openings in the Berlin Wall. However East Germans were not allowed to use these portals because their Soviet puppet government feared losing its population to the West.
Nowadays at the site of the old checkpoint is a museum and some souvenir shops. For this holiday season there is also a Christmas tree decorated with Ukrainian flags. It acts as a reminder that the threat of Russian imperialism is not just something in the past.
Vladimir Putin was a lieutenant colonel in the Soviet secret police (the KGB). In the 1980s he was stationed in East Germany. So in addition to showing support for Ukraine, this pro-Ukraine Christmas tree could be seen as a dig at Putin.
The flag of Ukraine is distinctive and simple. If you wish to add one or more to your own holiday decorations, the color codes can be found here.