Complex sentence: Many Khalkha nobles and folks fled to Southern and Northern Mongolia because of the war.Russia threated to exterminate the Mongols who fled to Northern Mongolia if they don't submit to Russia but many of them submitted to Galdan Boshugtu.
Split into simpler sentences: Many Khalkha nobles and folks fled to Southern and Northern Mongolia because of the war. Russia threatened to exterminate the Mongols who fled to Northern Mongolia if they didn't submit, but many of them submitted to Galdan Boshugtu.