This was stated by analysts at the Institute for the Study of War (ISW).

As ISW previously assessed, "Putin will very likely secure significant gains in restoring Russian suzerainty over Belarus regardless of the outcome of his invasion of Ukraine. Russia’s likely permanent gains in Belarus present the West with a decision about how to deal with the potential future security landscape on NATO’s eastern flank."

Analysts estimate that if the West allows Putin to maintain his current gains in Ukraine—particularly Crimea and eastern Kherson region — then the Kremlin will be able to use both occupied Belarusian and Ukrainian territory to further threaten Ukraine and NATO’s eastern flank. 

"The West could alternatively set conditions for a future in which a territorially-whole Ukraine becomes a robust military partner in defending NATO’s eastern flank against Russia and Russian-occupied Belarus."

The report also shows that the preferable long-term future is predicated on immediate and sustained decisive Western action to empower Ukraine to expel Russian forces from its territory. 

"It is extraordinal unlikely that the West will be able to defeat or respond effectively to the Russian campaign to absorb Belarus without first defeating the Russian invasion of Ukraine."