Past tense of combat meaning to have fought against or struggled with something.
From 'combat' with the past tense suffix '-ed' added. The double 't' in 'combatted' is the British spelling, following the rule that single-syllable or stressed-syllable verbs double the final consonant before adding '-ed.'
English spelling rules create 'combatted' with two t's in British English but 'combated' with one t in American English — it's the same reason 'admitted' has two t's but 'benefited' has one, depending on where the stress falls.
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