Culturally and politically diverse nations – France, Great Britain, the Soviet Union, and the United States – every one of the Allied Forces in World War II were united in their common fight against Germany.
One small thought: The subject or the subject phrase, after stripping of the modifiers from the clause (together with modifiers' commas, dashes or brackets) should not be separated from their verbs by a comma.
A. Culturally and politically diverse nations – France, Great Britain, the Soviet Union, and the United States – every one of the Allied Forces in World War II were ---
Here the clause started with culturally through the United States is a modifier. The real subject is 'the every one of the Allied Forces in World War II' which is singular and it verb 'were; is plural; this is a blatant SV error.
B. Each of the Allied Forces in World War II – France, Great Britain, the Soviet Union, and the United States – were culturally and politically diverse nations,--
Popping up SV error in 'each … were'. C. Culturally and politically diverse nations – France, Great Britain, the Soviet Union, and the United States – each part of the Allied Forces in World War II, --
a fragment as already has been pointed out.
D. The Allied Forces in World War II – France, Great Britain, the Soviet Union, and the United States – were culturally and politically diverse nations,--
The best choice.
E. Each one a nation with cultural and political diversity, the Allied Forces in World War II – France, Great Britain, the Soviet Union, and the United States – were ---
Here, the phrase starting from ' the Allied is a modifier and the actual subject is 'each one nation" with its verb 'were'; a very noticeable SV error.