There are two major problems with this sentence.
(1) A comparison should evaluate two parallel elements. The intention of this sentence is to contrast two World War II battles. However, the original sentence contrasts the Battle of Midway with historians regard the.... The correct contrast should involve two parallel elements: the Battle of Midway and the Battle of Debrecen.
(2) as of marginal importance can be written in a more concise manner (as marginally important). Further, the reference to historians can be omitted as it is clearly understood in the context.
A. A battle (Midway) is incorrectly contrasted with historians regard for a battle; as of marginal importance is wordy
B. The sentence correctly contrasts the two battles; the wordy phrase as of marginal importance is replaced by the concise phrase marginally important
C. A battle (Midway) is incorrectly contrasted with historians regard for a battle; as of being marginal importance is awkward, wordy, and grammatically incorrect
D. The wordy phrase regarded as being of relatively minimal importance should be replaced by the more concise phrase regarded as relatively unimportant
E. The wordy phrase regarded by historians as being relatively unimportant should be replaced by the more concise phrase regarded as relatively unimportant; the phraseby historians is clearly understood in the context of the sentence and does not need to be repeated