This is exact opposite of a Suzerainty.
A vassal state is the subservient/smaller state in a mutually obligatory political relationship. When a smaller, weaker nation cozies up to a larger, more powerful nation, it’s termed a vassal state, much like vassals were subjects of a kingdom’s power and protection in medieval times.
“Vassal” is literally Old French for “subject, subordinate, servant.”
Why I Looked It Up
In a news article about the invasion of Ukraine –
Ukraine is about the size of Texas, bordered by Russia to the east and a Pro-Putin, Russian vassal state of Belarus to the north.