The stamp act was the first interal tax levied directly american colonist by the british goverement. The tea act was that the colonist had never accepted the constitionability of the duty of tea, and the tea act rekindled their opposition to it.
The American colonists were justified in doing this simply because their colonies had become too big and too important to be treated as a colony by the British. The British should have given the colonies some autonomy, but they did not. The analogy I like to use is that of teens and their parents. Parents have to give teens more independence as they grow up. If they do not, the teens may justifiably rebel.
The British were not, on the whole, brutal or oppressive towards the colonists. However, they would not let the colonists have much in the way of self-rule. This had been fine when the colonies were still small and economically weak. By the 1760s and 1770s, however, the colonies were "teenagers." They were big and strong enough to expect some autonomy. When Britain reacted to requests for autonomy by being more strict, the colonists were justified in rebelling.
The city of Nanjing was extremely vulnerable to Japanese attack because the Chinese armed forces had been removed from the city.
