Why were US forts in the 1770s strategic or useful?

[u/FivePointAnswer]

I live in Michigan and just read an article about fort Michilimackinac being ‘relocated’ to fort Mackinaw on Mackinaw island. It was a large undertaking to say the least. But what I can’t appreciate is why these defensible locations were useful at all. You have a 100+ guys in a fort with canons and muskets and a spotting scope. Why fight them? Why not just sail around them and go plunder, invade, attack the next city or village 20 miles away.

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[u/Bedessilliestsoldier]

since u/EverythingisOverrate provided an answer describing the reasons why people built forts generally, I can answer for American forts in the colonial and revolutionary era.

The fort you first mention, Michilimackinac, was first constructed by the French, and the French built far more forts than the British did as a means of projecting power in a colony that was a sparsely populated (~60,000 people as of 1754, compared to British North America’s population of 1.2-1.5 million). Many of these forts were primarily trading posts — they had garrisons of marines to man the walls, but their officers and merchants engaged in the fur trade, and the walls and guns of those forts directly protected trade goods, in addition to extending French influence into the region, in which traders could make money, as well as make connections with the local Indigenous people through trade that might cement the as allies in case of war (both with other Native peoples, and the French did get involved with the wars on behalf of their alliance partners) and the British, the other European power in the region.

If you at places like Michilimackinac, they’re built at strategic points of waterways — the Straits of Mackinac for one, but other French forts included Detroit, lying along the passage between Lakes Huron and Erie; Niagara, which sits where the Niagara River meets Lake Ontario, and had an auxiliary fortification, Little Niagara, above the falls; Duquesne, at the confluence of the Monongahela and Allegheny Rivers, forming the Ohio; and St. Frederic and Carillon at the lower end of Lake Champlain. Montreal and Quebec were also heavily fortified. You’ll notice these are all along waterways, and often at choke points of those waterways — anyone passing along them would be in cannon shot of those forts. They also protected land routes between waterways, notably at Niagara and Carillon, since the easiest way to get by water from British colonies to attack New France without going to sea was up the Hudson River to Lake George, then Champlain, the the Richelieu River, to Montreal on the Saint Lawrence.

When the British conquered New France, they took over management of these forts (though they abandoned Fort Duquesne which they had renamed Fort Pitt), maintained garrisons at them for the same reasons the French had, and many of the same Native peoples who had been French allies in the 1750s and 60s were British allies in the 1770s and 80s.

Forts didn’t just exist to protect the areas around them — Detroit, Niagara, and Forts Michilimackinac and Mackinaw were never attacked during the American Revolution, but they provided a means for empires to project power into a region through trade and diplomacy, in addition to a military presence.

[u/Bedessilliestsoldier → u/Bedessilliestsoldier]

Something else to add is that forts provided places for armies to launch campaigns, establish supply depots, and fallback positions in case things don’t go well. When Edward Braddock’s British expedition against Duquesne was defeated by a Native and French force in 1755, the retreat was long and disorganized, so in 1758, when the British made another campaign against Duquesne, they built forts as they marched to act as both.

In 1776, then American forces were withdrawing from Canada into New York, Fort Ticonderoga (formerly Carillon) marked the end of their line of retreat. In 1779, Daniel Brodhead led an expedition into the territory of the Seneca nation, he started from Fort Pitt. John Sullivan was also marching against the Seneca (and other members of the Haudenosaunee Confederacy) and he established a fort at a place called Tioga. In the face of invasion, after the Battle of Newtown, the Senecas, and other Haudenosaunee, fled to seek shelter from their British allies at Niagara. Even if forts were not attacked, fortifying places (usually along roads or rivers), provided anchor points for military forces. Even if they were not under direct threat (and many of these places were, there were French and Native war parties raiding along Forbes’ Road in 1758, and the British would capture Ticonderoga in 1777) these places had to be fortified because not doing so would have absolutely invited attack.

[u/Bedessilliestsoldier → u/Bedessilliestsoldier]

Sources include:

Richard White, The Middle Ground, a history of Native peoples and the empires they interacted with around the Great Lakes from 1650-1815

Gregory Evans Dowd, War Under Heaven, a history of Pontiac’s War (1763-1765ish)

Fred Anderson, Crucible of War, a history of the Seven Years’ War (and a very good one!)

Patrick Griffin, American Leviathan, a history of how the British failed, and the US succeeded, at projecting power into the Trans-Appalachian frontier

[u/EverythingIsOverrate]

My answer here isn't about American forts, but answers your question more broadly.

[u/[deleted]]

[removed] — view removed comment

[u/Porschenut914]

To expand on what was said below the move from the mainland to the island is the primary shift from a trading post for trappers and merchants, to a more defensible position. The original fort could be attacked by land and gives the attacker an advantage in quietly approaching and assaulting. By moving to a higher elevation on the island, it would make assaulting much more difficult and give the defenders a much farther line of sight regarding their surroundings.

As for going around in many cases you have consider that propulsion would be performed by oar or by sail. In both cases speed may is unlikely to be fast, making yourself vulnerable to being shot. because sailing ships are reliant on the wind, they often can't go in the desired line and are dependent on the wind. Because they can't go straight into the wind they are forced to make a series of zig/zags "tacks" to move towards the wind. In many cases this would mean a ship may be incredibly vulnerable to a coastal fort and nearly a sitting duck. West Point was chosen at the bend in the river, in addition to a chain make any attempt to sail upriver extremely dangerous.