Thread by @bazaarofwar

FORTRESS MANHATTAN

It’s easy to overlook New York City’s military geography since it hasn’t faced a live threat in over 200 years. But it is perhaps the finest natural naval bastion in the world, one of the major reasons for its place as America’s economic capital.
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Any good naval base has traditionally had a few key elements: a defensible harbor, a large roadstead for assembling the fleet, and access to deep water. All major naval bases have this: Yokosuka on Tokyo Bay, Southampton on the Solent, Brest and Sevastopol Roadsteads…

The eastern seaboard of the US is endowed with plenty of these: Boston Harbor and Newport on the Narragansett; Wilmington/Philadelphia on the Delaware Bay, Norfolk, Baltimore, and Washington on the Chesapeake—even Charleston Harbor, although small.

But nothing comes close to New York. It has not one but two routes to open ocean, each with a large roadstead, multiple partitions that allow defense in depth, and surrounding geography that makes any approach from the land exceptionally difficult.

The harbor was originally on the southern tip of Manhattan. Even after it expanded to Brooklyn, attacking ships would have to pass through narrow channels between Governors Island, Liberty Island, Long Island, Ellis Island, & Manhattan, all of which were at some point fortified.

New York Harbor has since then spread to the entire Upper Bay, which exits to the Lower Bay via a mile-wide passage called the Narrows. This put ships passing through the middle within easy cannon range of batteries on either shore.

Beyond the Narrows is the Lower Bay, another roadstead which opens on the Atlantic via a 5-mile-wide channel protected by a long sandbar. Channels were dredged beginning in the 19th century, but larger 18th-century warships could not clear it at low tide.

At the opposite end of New York City, ships could sail up the East River to the Long Island Sound. The southwestern end of the Sound is a roadstead bounded by Davids Island and Sands Point at the entrance, with an even more defensible passage between Great Neck and City Island.

Large ships couldn’t always transit from the Lower Bay to the Sound—the East River passes through Hell Gate, a strait with difficult winds, currents, and shoals before blasting/dredging widened it in the 19th c. But smaller vessels could, including at a least one 50-gun ship.

How did this hold up in actual war?

Not well at first. Even the best ground needs men to defend it, and the Dutch could not hold New Amsterdam and its strong fort (pop. 9,000) against the English invasion in 1664. Nor could the English hold it when the Dutch came back in 1673.

Similarly, the Continentals lacked the numbers to fully exploit this geography. They were heavily outnumbered by the British expeditionary force sent in 1776, and completely lacked a navy with which to contest their entry into the Lower Bay.

The British handily took Staten Island, and Washington’s batteries on the Brooklyn side of the Narrows were too weak to stop them from sailing through. Only the newly-built fortresses guarding Manhattan deterred a direct attack—they instead attacked overland via Long Island.

These preparations delayed the British for two months, but they easily beat the Washington when they attacked on 27 August. After besieging his Brooklyn camp another 3 weeks, they marched around Washington’s flank and crossed to Manhattan, forcing him to beat a hasty retreat.

The British were able to make good use of the city, however. When a strong French squadron arrived on the Delaware in 1778 to aid the Revolution, the smaller British squadron fled to the protection of New York harbor.

The French pursued, but hesitated around Sandy Hook. The local pilot advised caution, and the ensuing delay for soundings gave the British time to set up a battery and form their ships up around the entrance—even with a much superior force, this was too great a risk.

The French then sailed off to aid the American siege of Newport, just past the entrance to the Long Island Sound.

The British used their interior lines and send 2,000 troops via the Sound to reinforce the garrison, and the siege ultimately failed.

New York remained a British stronghold for the rest of the war. Although Washington and Rochambeau contemplated an attack in 1781, the defenses were too strong; instead, they marched all the way to Virginia to win a more attainable victory with naval support at Yorktown.

The situation had changed by the time of the War of 1812. New York was defended by many more soldiers than Washington had, and several forts had been constructed around New York City (incl. Fort Wood, where the Statue of Liberty now stands).

Although the British blockaded both entrances to New York City, they never entered the Lower Bay or passed Sands Point, while shore batteries and shallow-draft gunboats harassed British squadrons that had ventured far into Long Island Sound.

The US began catching up with Britain—the strongest naval power—over the rest of the 19th century: coastal defenses got better as rifled artillery extended in range, accuracy, and power, while the US Navy grew in size.

This had the effect of extending NYC’s defensive perimeter around the entire Long Island Sound. Batteries on islands and nearby shores could range the entire entrance, practically turning the Sound into a gigantic roadstead, while also making it hard for subs to enter.

This was evident in both World Wars. Although German submarines sank staggering quantities of Allied shipping off the coast during the 1940s, they avoided Long Island Sound and the Lower Bay altogether—the closest they got was the waters off Newport.

But even supposing America faced a much more capable naval adversary in the 20th century, their options to attack Manhattan wouldn’t be good. The easiest would be to land up or down the coast and attack by land.

That still requires a blockade of the Lower Harbor or Long Island Sound, and then it’s at least 50 mi through New Jersey over very marshy terrain or 120 mi across Connecticut. Or by Long Island, but at ~15-mi wide it’s very defensible and vulnerable to naval fire from the Sound.

Failing that, something more complex would be needed: amphibious landings to knock out shore defenses, allowing ships to move into the Sound or Lower Bay, which can then provide naval support—like what the Germans did against the Russians in Operation Albion.

The missile age renders all these considerations even more complex, just one of many reasons it's hard to seriously entertain any military threat to New York in the 21st century.