Historeography of imperial Russia and later Soviet union was undermined by ideology. Such ideology exists to these days in Russia.

Territories of modern day Belarus were never under Mongol or Tatar rule. After much of territories of present day Russia and Ukraine fell to Mongols, territories of Belarus joined the Grand Duchy of Lithuania . The first and only Lithuanian king Mindaugas was crowned in Navahrudak (western Belarus). After Mongol invasion Ruthenian Boyars (ancestors of Ukrainians) leaned towards Lithuanian dukes to escape Mongol rule.   In the battle of Blue waters (1362) grand duke Algirdas defeated Golden Hurde and much of Ukraine Kyiv principality, parts of Podolia and Volyn were incorporated into the GDL. Although, Lithuanian grand dukes continued paying some tributes for
southern territories to Tatars to escape their incursions from Crimea
into neighbouring territories. Some territories in eastern Galicia were in the sphere of Poland. The Grand Duchy consisted of Baltic and Slavic military units, who could successfully resist Tatars and Mongols, while Muscovy couldn’t remaining Hurde’s vassal state till late 15th century.  In other words we fought Tatars successfully, while you couldn’t. We had a state with Gediminas bloodline. The state religion was Orthodoxy. The official language of the state was Ruthenian. Grand dukes were entering dynasty marriages accepting Christianity keeping both pagan and Christian names to negotiate between two communities – pagan and Christian. Our problems began when Musovy Vasily III and his Ivan IV began incursions into our territories burning and looting our cities in early 16th century. Then our rulers entered negotiations with Poles in Lublin giving up some southern territories (some territories of present day Ukraine) . Thus Polish-Lithuanian Commonwealth was formed in 1569 due constant threat from Muscovy.