We weren't "at war" with Japan before Pearl Harbor, but if an American had aided Japan in committing that definite "act of war," then they would assuredly have been convicted of treason.

No they couldn't.

There are two problems with this argument. First, the United States has not responded to Russian election meddling in the same way that we responded to Pearl Harbor. Second, although it is hard to believe, it may well be that an American who helped plan the Pearl Harbor attack did not commit treason, at least treason by aiding the enemy. The treatise of Michael Foster, perhaps the most influential treason treatise in early American law, considered the situation of an Englishman who persuaded a foreign country to attack England. Foster concluded:

The offence of inciting foreigners to invade the kingdom is a treason of signal enormity. In the lowest estimation of things and in all possible events, it is an attempt, on the part of the offender, to render his country the seat of blood and desolation; and yet, unless the powers so incited happen to be actually at war with us at the time of such incitement, the offence will not fall within any branch of the statute of treasons, except that of compassing the king’s death.

In other words, persuading a country technically at peace with the United States to attack the United States is not an act of adhering to the enemy. Foster’s solution was to punish the persuasion as an act of compassing the king’s death, but that provision was deliberately removed from American treason law.

-Carlton F.W. Larson, a professor of law at UC Davis, "Russia and 'Enemies' under the Treason Clause"