According to the terms of the Tripartite Pact, each Axis nation was only obligated to come in to defend their allies if they were attacked. Since Japan did the attacking, Hitler was under no obligation to also declare war on the US and neither was Mussolini (who only did it because Hitler did anyway.)

Interestingly, Japan did not declare war on the Soviet Union when Germany invaded it and Stalin did not declare war on Japan until almost the end.

If the US did declare war officially it was only after Germany declared war against them and it was certainly after Roosevelt's famous day that will live in infamy speech.

The Germans clearly did not like lend lease but they too were obtaining arms and supplies from neutral nations closer to home - iron ore from Sweden for example. US escorts to Iceland were a good conveying procedures exercise and in no way involved a direct act of war against Germany.

Alas, (just as in WW1) the German submarines considered them fair game in such a circumstance as the Reuben James incident shows.

I wasn't impressed by The Life and Death of Colonel Blimp myself - it was based upon the cartoon character created by David Lowe in the pre war 1930s as a satire on the establishment and didn't transcribe into a good war movie.

http://en.wikipedia.org/wiki/Colonel_Blimp