america is (presently) the worlds most powerful country. and means it has obligations and complications that other countries don't have, and of course it has to balance these with what is best for itself.
russia is not america's biggest threat, but russia does align itself with 2 that are, china and iran.
america needs to retain a relationship with russia that is not 100% adversarial, as it will need favours from russia to help in containing or destroying iran, and as a wedge with china.

and next of course on trumps "to do" list is iran,

It is called realpolitik.