Complex sentence: The interests of Western powers (particularly Russia and Britain) in these areas were guaranteed by treaties with the Qing dynasty which the Republic of China pledged to uphold and by recognizing the independence of Mongolia or Tibet, the Western powers would have invalidated those treaties.
Split into simpler sentences: The interests of Western powers (particularly Russia and Britain) in these areas were guaranteed by treaties with the Qing dynasty which the Republic of China pledged to uphold. By recognizing the independence of Mongolia or Tibet, the Western powers would have invalidated those treaties.