Belarus' history has been less of a single nation's story than as part of a regional history that involved the Belarusian people.
Belarus (formerly known as Belorussia or White Russia) was colonized in the 5th century by Slavic tribes,replacing the earlier Baltic tribes which had settled in the region in the 3rd century- it was a relatively peaceful gradual assimilation of the Balts into the Slavic culture.(the other two Slavic republics included in the Soviet Union were Russia and Ukraine)
In the 9th Century,principalities arose in this area. One of those principalities was the Principality of Polatsk was the first independent state on the territory of Belarus.
When the Mongols invaded the region in the 13th century,Belarus was one of the few areas that escaped being destroyed.
I have seen conflicting information on the following statement - most sources I've read say Kiev Rus' (future Russia) effectively ruled over the Belarus region from the 9th century to the 12th century. Others say there was only a 20 year period that the Principality of Polatsk was a vassal of Rus'. It may be that everything in Belarus was under the rule of Rus' except the Principality of Polatsk.
For 200 years,starting in the 13th century,the Grand Duchy of Lithuania rose to power. The Belarus region was part of that entity,which included Lithuania,Belarus, Ukraine and the Russia strip from the Baltics down to the Black Sea,partially Poland,and Latvia.
In 1386,the rulers of Poland and the Grand Duchy of Lithuania married. Leading to eventually forming a federated state called Polish-Lithuanian Commonwealth (Rzeczpospolita) in 1569.
Note - starting in 1486,the Russians started a series of military campaigns with the aim to incorporate the old lands of Kievan Rus',specifically Belarus,areas of Russia not already conquered and Ukraine.
In 1696,the Belarusian language was outlawed and Polish became the official language of the Commonwealth.
From this date until 1795,Belarus experienced a turbulent period in their history because of two long wars (both of which involved Russia). These wars weakened the federated state and it lost its independence when Poland/Grand Duchy entity was divided in 1795 between Russia,Austria and Prussia. Belarus became part of the Russian Empire.
In the 1840s,Russia instituted a policy of Russification in Belarus (and de-Polonization). The Belarusian language was prohibited in public schools,there was a campaign against Belarusian publications,and pressure to force Roman Catholics to convert to Orthodox faith.
There was a revolt in 1863 against the suppression of the native language and culture and a further suppression of the Belarus written language was enforced (until 1905).
1906 through 1916 saw massive deportations of Belariusian peasants (33,000) moved off farmland to Siberia and replaced with Russians.
1914 - 1918 - WW1
The Russian Revolution happened in 1917.
Belarus declared its independence from Russia in 1918. Russia's response was to sent in the Russian army.
The Polish-Soviet War (1918-1921)saw west Belarus(almost half of its territory) go to Poland and the larger eastern part go to Russia.
Poland instituted a Polonisation policy in the eastern section of Belarus now under her control.
1932/33 saw famine in Belarus caused by the Soviet's introduction of collective farming/ Soviet economic policy.
1936- 1940 - The Great Purge. More than 86,000 Belarusians suffered political oppression and over 28,000 were put to death at the Kuropaty camp near Minsk.
As part of the secret Nazi-Soviet Nonaggression Pact in 1939,Russia got to take back western Belarus from Poland.
Belarus was occupied by the Nazis in WW II from 1941 until Belarus was liberated by Soviet troops in 1944 and "was one of the war's most devastated battlefields". 25% of the population died during WW2. WW2 deaths combined with deaths from the Soviet purges,Belarus' population was a third of what it had been in 1930. It was not until the 70s that its population returned to previous levels. Before the war,Belarus had a large Jewish population,but almost the entire Jewish population was exterminated by the Nazis.
The Germans established the Minsk ghetto for Jews within the first year of occupation and started a program of mass executions of Jews.