 Complete the following phrase:  As winter approached, the Russian army was forced to leave Shumla and retreat back to Bessarabia. In February 1829, the cautious Wittgenstein was replaced by the more energetic Hans Karl von Diebitsch, and the Tsar left the army for St Petersburg. On 7 May, 60,000 soldiers led by Field Marshal Diebitsch crossed the Danube and resumed the siege of Silistra. The Sultan sent a 40,000-strong contingent to the relief of Varna, which was defeated at the 