Combat, the noun, has the stress on the first syllable.
First, military training and exposure to combat does not create the wacko battle-scarred soldier so often depicted by Hollywood, nor does it translate into criminal behavior. – Albuquerque Journal
Combat, the verb, has stress on . . . either syllable. It is tempting to get all purist here and insist on stressing the second syllable in the verb, but both forms have been widely adopted.
A beefed-up corporate law enforcement unit, a new anti-fraud agency and more efficient criminal cases are among a suite of measures to be introduced to combat white-collar crime. –Irish Examiner
For combat, the verb, the stress distinction may be heading for extinction.