The r/AskHistorians twitter alerted me to an answer on Chamberlain's policy of appeasement at the time, and it mentioned the US and Roosevelt briefly, which got me thinking. Was the US involved in the build-up to the war? What were its reactions to the actions of Germany (rearmament, the Anchlusss, the Sudetenland, etc.) and to what extent was it communicating with Britain, France and other powers?
Answered here: https://www.reddit.com/r/AskHistorians/comments/1wx6w9/would_the_us_have_still_gone_to_war_with_germany/cf6g970 by /u/coinsinmyrocket
TL;DR - FDR was for entering the war and the US provided aid to the allies, but the US had strong isolationist ideals at that time.