Belarus is a satellite state of Russia and therefore a loyal ally. Its strategic position is excellent when it comes to attacking Ukraine. A succesful attack on Ukraine’s northern flank could cut off critical western supplies flowing into the country. Why does Belarus not strike?

1. The army of Belarus is very small. It has a core of about 15,000 soldiers. It’s highly unlikely they would be an effective fighting force.
2. There is a lot of dissent in Belarus. Just like in Russia itself going to war against Ukraine could unleash the dissenting powers within Belarus. The regime could be toppled from within if it sent its army into Ukraine. Open warfare against Ukraine would be very unpopular among the population of Belarus.
3. The Ukrainians are well aware they could be attacked from that direction, so they have prepared for this eventuality. If the Russians can’t break through there is no reason to believe the army of Belarus could do any better.
4. By officially being at peace with Ukraine the territory of Belarus functions as a safe zone for Russian soldiers. The Russian army can move around there without risks. As long as Belarus doesn’t attack Ukraine the Russian army operating there cannot be targeted. Belarus is more useful to the Russians by officially staying out of the war.
5. Belarus has nothing to gain from such a move. Even a successful attack would be very costly. The reaction of the west would likely be very severe. Belarus would wreck its international reputation even more and get little in return. Belarus is already getting got deals from Russia for the pro-Russian role it is playing right now. On the whole Belarus is being smart and playing its cards well by NOT attacking Ukraine.