Whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere, is guilty of treason and shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United States.
Trump is currently giving aid to Russia by letting one of our allies out to dry. An ally that Russia is current occupying.
Before you go there, a declaration of war is not necessary to consider another country/group an enemy of the United States. For example, interfering in our elections with the intent of putting a Russian agent in the Whitehouse would quickly put a country on the enemy list.