You are General Sir Alan Brooke, Commander-in-Chief Home Forces. It is August 1940. The German Army stands twenty miles from Dover. You must defend England against Operation Sea Lion — the German invasion of Britain.
Your objective is to hold London and maintain the political will to resist until autumn storms close the Channel. You have 12-hour turns across a 90-day campaign. The Germans need settled weather. Time is your ally — if you can survive.
Map — The operational area. British units show Union flag markers, German units Iron Cross. Five defensive lines are marked: Coastal Crust, Corps Stop Line, GHQ Stop Line, North Downs Ridge, and Outer London Defence Ring. Strength bars appear below each marker. Unknown German formations show as ??? until identified.
Left panel — Situation Report, Royal Navy Channel Force, ULTRA/Bletchley Park intelligence, meteorological conditions, and the War Diary. Read the diary after every turn — it records all significant events.
Right panel — Click any unit on the map to view its full dossier: commander, formation history, combat effectiveness, supply state, and formation dossier with real subordinate units. Issue orders to British units from here.
Turn Summary — After each ADVANCE a panel slides up summarising what changed: weather, morale, RAF strength, supply, German supply, and the latest intelligence entries. Auto-dismisses after 8 seconds or click to close.
ADVANCE 12 HOURS — Each click advances half a day. All systems resolve: movement, combat, supply, intelligence, naval operations, air battle. Read the turn summary and diary after each advance.
Watch the meteorological panel. The Germans need four consecutive GO windows — two days of settled weather — before they will launch. The window pips show recent history. When three consecutive GOs appear you should be moving units to forward positions.
Watch the WINDOW STATUS counter in the footer. It tracks consecutive GO turns in real time — amber at 1–2, red and pulsing at 3, critical at 4. At GO 3/4 you should already have forward units at maximum readiness. At GO 4/4 the invasion launches.
ULTRA will warn you when German radio traffic falls silent — the pre-operational pattern preceding every major offensive. Three turns of radio silence means S-Day is imminent. When the invasion launches, CROMWELL is issued and the BBC auto-broadcasts an emergency bulletin.
Autumn works in your favour. By turn 80 storm probability rises sharply and a diary entry will confirm the window is closing. Every storm is a reprieve. Every GO turn is a crisis.
Click any British unit to select it. The right panel shows strength, supply state, combat effectiveness, zone, current orders, and a collapsible formation dossier. Coloured dots on markers show current orders: blue = HOLD, amber = WITHDRAW, red = ATTACK, green = REST, purple = MOVE.
All orders have a command lag of 1–2 turns before taking effect. A unit ordered to MOVE on S-Day will not arrive for two turns. Factor this into your planning.
Combat Effectiveness is a composite of strength, supply and fatigue. A well-supplied, rested unit at full strength fights at full capacity. A tired, undersupplied unit at 40% strength fights at a fraction of that. Check the effectiveness rating before committing a unit to battle.
The battlefield runs south to north across five zones. Germans land in the Beach Zone and drive toward London. You decide how hard to contest each defensive line.
The Battle of Britain runs in the background throughout the campaign. RAF strength is shown in the turn summary and sitrep. If it falls below 35% the diary will issue an Air Ministry warning. Below 20% Fighter Command is effectively broken and the RN becomes dangerously exposed.
Three decision events give you direct control over RAF tactics:
Four RN flotillas are shown on the map: 19th Destroyer Flotilla (Sheerness), 5th Destroyer Flotilla (Portsmouth), Channel Force Heavy — HMS Revenge and Resolution (Plymouth), and MTB Flotilla 24 (Dover). Click any RN marker to view its status.
Pre-invasion the RN patrols and harasses barge concentrations in French ports. Once the invasion launches, destroyer flotillas sortie every night, attacking the transport fleets. MTBs clear the German E-boat screen first. The more intact your RN, the more you starve German supply.
A decision event will ask whether to commit the capital ships. Their 15-inch guns could devastate the barge fleets — but Luftwaffe dive-bombers sank ships in the Channel before, and losing HMS Revenge in the Straits of Dover would be a strategic catastrophe.
German units start as unknown formations (???) on the map. They are progressively identified through ULTRA decrypts, aerial reconnaissance, and combat contact. The ULTRA panel shows Bletchley health, how many German units have been identified, and whether German radio silence is detected.
Bletchley health degrades if Luftwaffe bombs signals infrastructure or if you act on intelligence in ways that might reveal the source. Three decision events involve ULTRA directly: whether to act on a decrypt and risk burning it, whether to use agent TATE for disinformation, and the capital ships question.
When Rommel's 7th Panzer is identified, you will receive a special ULTRA FLASH. This is the most dangerous unit on the board — treat it as your primary threat.
The Germans are not scripted. They respond to what you do. In the first two days they consolidate the beachhead. Then they probe for weaknesses in your line. When they find a gap — or create one — the panzers exploit it at speed.
Rommel's 7th Panzer will seek gaps and drive through them regardless of flanks. Infantry divisions are more methodical — they consolidate when facing strength and advance when the path is clear. If you WITHDRAW a unit, expect immediate German pursuit. If you leave a unit on REST in a forward position, expect it to be probed.
If the Germans stall without breaking through by day 12, they enter a desperate phase and drive straight for London regardless of cost. The invasion timetable was always tight. They know it too.
When a British unit takes heavy casualties the war diary will say so clearly — "HEAVY CASUALTIES — 3rd Infantry Division: strength 87% to 52%." When a unit is shattered you will know immediately. Watch the turn summary after every advance for the damage report.
At critical moments GHQ will present a command decision. These are not simple choices. Each option has real mechanical consequences. There are up to nine decision events covering: RAF tactics (Park vs Leigh-Mallory, Biggin Hill, beachhead air support), ULTRA intelligence (act on decrypts, TATE double-cross), Royal Navy (commit capital ships), and land battle decisions (artillery deployment, London reserves). The game cannot advance until you decide.
Read the situation carefully. There is no correct answer — only better and worse judgements given the circumstances at that moment.
Victory — Survive to turn 120 with London held, or destroy the German beachhead entirely. Autumn storms will close the Channel and end the campaign.
Defeat — any one of these ends the simulation:
Operation Sea Lion was cancelled by Hitler on 17 September 1940, primarily because the Luftwaffe failed to achieve air superiority. The Royal Navy remained intact and would have inflicted catastrophic losses on any invasion fleet — Admiral Raeder told Hitler the Channel crossing would be a massacre. This simulation asks the question history did not have to answer: what if the weather had held, the RAF had broken, and the order had been given?
For a full reference of all terms, zones, orders and systems, open the GLOSSARY tab above.
Good luck, General. Britain is in your hands.
Situation briefing...
BBC HOME SERVICE · NINE O'CLOCK NEWS · 14 AUGUST 1940