Upvote:5
The timing was much more connected to the Russian February Revolution than to the US Declaration of War.
It takes time for such operations to be planned, organized, and carried out; it is unthinkable that it could have been done in few days. Moreover, it made an eminent sense for the Germans to send the Russian Boleshevik exiles back to Russia regardless of any US policy.
See the answer to another recent question for more details.