A world in which Charles Lindbergh is president in 1939 (we're not talking about him being elected in November 1940--even that is very unlikely but not quite inconceivable) and Oswald Mosley is British Union of Fascists Prime Minister in 1939 (we're not talking about him becoming Labour Prime Minister--even that is very unlikely but not quite inconceivable) would be a world totally different from our own and totally different from our own well before 1939. (In OTL the BUF didn't even run candidates in the 1935 General Election; and FDR was of course unbeatable by Lindbergh or anyone else in 1936, even if Lindbergh had the slightest desire to run that year, which he didn't AFAIK.)
And yet we assume World War II breaks out at the same time and in the same way (Hitler invades Poland in September 1939...) as in OTL! I'm getting a little bit tired of pointing out the problem with "drastically change the world long before major political event X but assume event X will be as in OTL" posts, but someone has to do it...
I agree. That's why I questioned of the french would declare if Hitler would invade Poland (him invading Poland is likely. although it may be a a different time). I wanted to expand on that, but
@NOMISYRRUC already made some points I wanted to make. With the UK not allied to France, France would seek another ally, because otherwise they'd be screwed between the fascist states of Germany, Italy, Spain and the UK. They'd be very nervous about that. And the USSR comes into the picture for them.
A french-russian alliance might be interesting for Poland to join, although they wouldn't like the idea of russians on their soil. Such an alliance might make Hitler hesitant about invading Poland.
Even though Poland wouldn't be eager to have an alliance with the USSR, after Hitler annexing Austria, Sudetenland and Tchechoslovakia*, they will know Danzig is the next step, so they will consider a russian alliance, especially if the french are allied with Russia.
*I'm pretty sure these will happen, although they may happen earlier than OTL, depending when Mosley came into power.