There are doubts as to who's side most Belarus troops would be on. There are already some in Ukraine fighting against the Russians https://www.dailymail.co.uk/news/article-10581717/Belarusian...

See also "Defections and resignations in Belarusian military prevented country from joining Ukraine invasion'" https://www.israelhayom.com/2022/03/07/defections-and-resign...