How to play Operation Sea Lion

What the game is

Operation Sea Lion is a real-time strategy game of the German invasion of Britain planned for September 1940, which never sailed. Here it can. You play the Wehrmacht or Britain, and the side you don't pick is run by an AI that gives the same orders a player can.

The campaign runs 80 days, from 13 August (Adlertag) to 31 October 1940, on a map from Cornwall to the Dutch coast. At normal speed that's about half an hour. Play it free in your browser at https://operationsealion.earthablaze.com/play on a desktop, tablet or phone. There's nothing to install and you don't need an account.

Choosing a side

Britain (Home Forces). Hold the air with Fighter Command, burn the barges in their basins, sink them in the Channel at night and throw any landing back into the sea before the autumn gales end the season.

• You win if 31 October passes with no viable German lodgement, or the beachhead is destroyed.
• You lose if the War Cabinet's resolve breaks, or London falls.

Germany (Army Group A). Win the air over the Channel, choose S-day on the tides and the moon, get nine divisions across in towed barges and keep them supplied until Britain gives in.

• You win if London falls, British resolve breaks, or a lodgement with a port and three divisions' supply holds to 31 October.
• You lose if the High Command loses confidence, the beachhead is destroyed, or S-day never comes.

The scoreboard: tonnage

One number keeps the score: the tonnes of supplies landed on the beachhead each day against what it needs. A division ashore eats about 300 tonnes a day sitting still, and two or three times that in a fight. The Germans need the figure high. The British need it low.

After S-day the Tonnage tab shows what's landed against what the divisions ashore need. Below 100% they go hungry; below 50% they can't attack. A gale that swamps one night's barge trains can matter more than a won battle.

S-day, weather, tides and the moon

Choosing S-day as Germany, or guessing it as Britain, is the biggest decision in the game. Rhine barges swamp above sea state 2. The crossing wants a calm night, a moon to keep station by and a rising tide at dawn on the beaches. After about mid-October the weather mostly shuts the Channel.

As Germany, press P to open the OKW planner. Its window chart shows sea state, tide at dawn and moonlight for the next fortnight. The fleets need ten days' notice, and S-day can't come before 12 September. The planned landing beaches are B to E, between Folkestone and Eastbourne.

As Britain, CROMWELL waits in the Home defence drawer on the Desk. It raises readiness across Home Forces, at a cost to production and nerves. Too early costs production; too late costs the beaches.

Air, sea and land

The three fights depend on each other. The Luftwaffe has to hold the air or the Royal Navy owns the Channel. The navy has to be kept out or the barges sink. The barges have to keep coming or the army ashore runs dry.

The big moments can be fought yourself in real time or left to resolve on the map. See Sea actions, raids and land battles.

Your first campaign

Each side starts with a short checklist that moves on as you do each thing. Press Skip to hide it, or Show the opening checklist again on the start screen to bring it back.

• Britain, the duty officer's checklist: set Fighter Command's posture (Conserve, Balanced or All out), keep Bomber Command on the invasion ports, put a destroyer flotilla on night patrol, open the Cabinet War Rooms and plan a raid on the barge basins, look at the Tonnage tab, then start the clock.
• Germany, the chief of staff's checklist: aim the Luftwaffe at radar stations and RAF airfields, look at the invasion ports, open the OKW and choose S-day, send an S-boat or minesweeping flotilla to work, look at the Tonnage tab, then start the clock.

Decisions arrive on the Desk as the days pass. Real commanders appear on the decision cards, which paraphrase what was argued and never invent quotations.

Gas

Mustard gas is in the game, because both sides planned for it. Using it is powerful and costly: opinion abroad, splits at home, civilian casualties and retaliation in kind. As Britain, only the War Cabinet can release it, by decision. Casualties are reported as numbers, never shown.

Keys and speed

• Space pauses and restarts.
• P opens the planner (the War Rooms button does the same).
• Esc closes panels.
• The speeds are 1×, 2×, 4× and 8×.

Real names, with care

Divisions, squadrons, ships, aircraft, places and commanders are real, as planned for Sea Lion and as they stood in 1940. German units carry the Balkenkreuz and British ones the RAF roundel and the White Ensign. There are no swastikas anywhere, and no party symbols or slogans. Waffen-SS formations are left out of the order of battle.

It's a counterfactual. Most historians think Sea Lion would have failed, mostly at sea, and the game lets you test that rather than assume it.