A British Major and aristocrat must team up with a vulgar guerilla leader in order to blow up a bridge in Nazi occupied Greece. Based on a true story.

Maybe something like:

The true story of an aristocratic British major and a vulgar guerrilla leader who team up to choke off supplies to Rommel's army in North Africa by destroying strategic railroad bridges in Nazi occupied Greece.
(35 words)

I tweaked it a little because given how mountainous the landscape of Greece is, I imagine that there were many bridges that would have been targets for sabotage.? If, however, they teamed up only to target the most important one, the one that would be hardest for the Nazis to rebuild, then amend accordingly.

fwiw

Maybe something like:

The true story of an aristocratic British major and a vulgar guerrilla leader who team up to choke off supplies to Rommel's army in North Africa by destroying strategic railroad bridges in Nazi occupied Greece.
(35 words)

I tweaked it a little because given how mountainous the landscape of Greece is, I imagine that there were many bridges that would have been targets for sabotage.? If, however, they teamed up only to target the most important one, the one that would be hardest for the Nazis to rebuild, then amend accordingly.

fwiw

DPG's re write is very good, I'd add that it could change the outcome of the war.

Based on a true story, an aristocratic British major and a vulgar guerilla leader together must blow up a bridge, in Nazi occupied Greece, that could change the outcome of WW2.