Final answer:
The incorrect tense in the passage regarding The Great Wall of China is 'will divide', which should be changed to 'was divided'.
Step-by-step explanation:
The word or phrase that is in the incorrect tense in the passage about The Great Wall of China is 'will divide' in the sentence 'The initial sections of the Great Wall were built when China will divide into many small states.' The tense of 'will divide' should be in the past to be consistent with the rest of the passage, changing to 'was divided' to accurately reflect that the division occurred in the past.