First, there was the common origin. The 13 colonies started out as two joint-stock companies granted a charter in 1606: the Virginia Company of Plymouth and the Virginia Company of London, also known as the London Company and the Plymouth Company. Their granted territories would take up much of the North Atlantic coast and over the next 150 years would be divided and extended to become the 13 colonies.

Those divisions and extensions were specified and defined by English monarchs and governments who had a very vague idea of the actual geography of their colonies, so that the actual borders had to be worked out on the ground, like neighbors who've been sold badly-surveyed adjoining properties. For example, Maryland and Pennsylvania were both given some of the lower Delaware valley. That made it necessary to interact. As these became English colonies ( after New York and the Delaware River were taken from the Dutch and Swedes) , that interaction involved English law, English intervention. And as they were contiguous, of course there was commerce between the colonies as well.

Lastly, the Seven Years War started in Pennsylvania, as a result of British colonists coming up against French claims. The colonies were quite divided in how to fight in the earliest stages of that war and not too effective, but when eventually it was won by the arrival of British professional soldiers there was suddenly a very large addition to the English territory: Canada. That territory came with administration costs- notably, a real standing army, instead of the simple volunteer militias that had done so poorly in the war. That war also brought the Proclamation of 1763, which set a western boundary to the colonies and denied them the territory they'd hope to gain. Resentment over the taxes to pay for the new territories, and resentment at being denied western expansion, were the common grievances that drove the 13 colonies to revolt.

There was extensive trade with the Caribbean: notably, most slaves for Virginia plantations came there first. When the revolt began, the Continental Congress wrote to the Caribbean colonies like Jamaica inviting them to join. They did not. Nor did the colonists succeed in carrying the revolt there with their few naval expeditions . It would have almost certainly been a bridge too far: the North Atlantic colonies were quite insignificant to the economy of England compared to the sugar plantations in the Caribbean, and England would have contested them much more vigorously. And those sugar plantations were more closely tied to England: their sugar was exported there, a British army was in residence to prevent a slave revolt, and the goal of every Jamaican colonist was to get a profitable plantation going, put it in the charge of an overseer, and go back home to England to live on the proceeds, become a Nabob.

The Continental Congress also hoped to convince French Canadians to join. There was an expedition north by Benedict Arnold in 1775. Quebec City was besieged: but did not fall,and the army was defeated, Arnold injured. And the Continental negotiators found they had little to offer that really interested the Quebecois. The invasion was a disaster.

But given the fact that the slave economy of colonies like North Carolina and Virginia had much in common with, say, Jamaica, why didn't the revolt stay limited to colonies like Massachusetts? The British government expected this to be the case. There were, it knew, far more Tories in the south. When it became impossible to suppress the revolt in Massachusetts and New York, General Henry Clinton did try to move into the south, hoping to have more success suppressing it there. However, Clinton ( and before him, Virginia Governor Dunmore) made the mistake of offering freedom to all slaves who would sign onto the British army. This alienated many otherwise sympathetic Tories. And those Tories who did try to help the British discovered that, though their help was welcomed, it was impossible for the small British armies to actually occupy and control an entire area and so prevent retaliation against them by Continental sympathizers.

Don Higginbotham: The War for American Independence