How can you stop your neighbor's cat from being mean to your kittens?
Unrelated cats are instinctively hostile toward kittens that are not theirs. There are some exceptions, such as when nursing mother cats encounter kittens that are not theirs, or when the cats are around one another enough that they share common scents and lose their unfamiliarity.
The reason why the US intervened in Vietnam was to stop the flow of communism and to stop what was called "The Domino Affect" which was basically a predicted hypothesis that would mean that after one country became communist, its neighbors would. By intervening in Vietnam they could stop the spread further down and try and turn them democratic.