What is the word for an array of nonaligned or friendly states that cushion or protect a larger country from invasion, as happened with the former soviet union and its eastern european neighbors?

The word for an array of nonaligned or friendly states that cushion or protect a larger country from invasion, as happened with the former soviet union and its eastern european neighbors is the buffer zone. It is created to specific areas that needs to be protected such as restrictions on the use of resources of that area and the country do its best to create measures to improve the conservation value of that area.