Complex sentence: On December 29,1911 Mongolia declared independence from the Qing dynasty and the National Revolution of 1911 ended over 200 years of Qing rule, but it had to struggle until the Revolution of 1921 to firmly establish "de facto" independence from the Republic of China.
Split into simpler sentences: On December 29,1911 Mongolia declared independence from the Qing dynasty; the National Revolution of 1911 ended over 200 years of Qing rule. The nation had to struggle until the Revolution of 1921 to firmly establish "de facto" independence from the Republic of China.