Bulgaria and Russia's relations continued to fall as Bulgaria accused Russia of meddling into its internal affairs, a fact that contributed to growing alliance between Tsarist Russia and Serbia. This led to antagonism between Bulgaria and Russia even when Russia motivated Bulgaria to form an alliance with Montenegro, Serbia and Greece to drive the Austrians away. When Bulgaria got a chance to occupy Constantinople during the First Balkan War against the Ottomans, Russia opposed Bulgarian military actions. Russia also refused to condemn Serbia and Greece for attacking Bulgaria in the consequent war. But Tensions between Bulgaria and Russia eventually erupted in the World War I when Bulgaria sided with German Empire over the promise to regain its lost soil. Eventually, Bulgaria and Russia suffered heavy military losses for their wars, and Bulgarian–Russian relations severed. It was not until 1930s that saw Russia, as Soviet Union, established relations with Bulgaria.