> Both words have identical meanings

Only online. The Viet Cong weren’t targeting civilians. They hit American soldiers. Targeting military and narrow political power are the hallmark of proper rebellions. Targeting civilians is how terrorism works.

The closest analog to terrorism in warfare is actually strategic air campaigns. Dresden and the London Blitz are closer to terrorism than e.g. the uprising in Bangladesh or even the Taliban toppling the Karzai regime. And lo and behold, strategic air campaigns have a history of uniting the enemy much more than undoing them.