Montesquieu

When your neigh­bor is a state in decline, it would be a mis­take to has­ten its ruin, because you are in this res­pect in the most for­tu­nate situa­tion that can be hoped for, there being nothing so conve­nient for a prince as neigh­bo­ring ano­ther who absorbs for him all the blows and cala­mi­ties of for­tune. And it is rare that by conque­ring such a state your real power is increa­sed as much as you have lost in rela­tive power.