one reason is to cut off the sale of cheap gas to europe frim russia, to keep a united europe from stealing thunder from the us. another is the decades-spanning plan to break up russia, which is a continuation of the cold war (it never stopped) another is to use ukraine as a testing ground for modern weapons and tactics. the us has been fighting insurgents for a long time and needs to update its models. another is the fight to keep russia from having a port which doesn't ice over part of the year. there's lots of reasons for this conflict.
United States