The problem is with your chosen pronouns. Observe how smoothly it flows once a few corrections have been made:
China has as much claim to North Korea as it does over Tibet. That is, if it does decide to invade and occupy North Korea.
Why did I choose the pronoun it? Quite simply because you said China has. You should maintain consistency among your pronouns, so if you use the singular "has", you are treating the noun (China) as a single, collective unit. That is, you're talking about China as the country in general as a single entity.
If you'd prefer to use the plural pronoun they, then the change is as simple as replacing "has" with "have" and "China" (singular) with "The Chinese" (plural):
The Chinese have as much claim to North Korea as they do over Tibet. That is, if they do decide to invade and occupy North Korea.