Passage: By the middle of the 14th century, the power of the Mongols was declining, and the Grand Princes felt able to openly oppose the Mongol yoke. In 1380, at Kulikovo on the Don River, the Mongols were defeated, and although this hard-fought victory did not end Tatar rule of Russia, it did bring great fame to the Grand Prince Dmitry Donskoy. Moscow's leadership in Russia was now firmly based and by the middle of the 14th century its territory had greatly expanded through purchase, war, and marriage.

Question: When did the power of mongol start declining?
middle of the 14th century