On 16 July 1940, Adolf Hitler issued Directive No. 16, ordering preparations for a possible German invasion of Britain. The operation was given the codename Sea Lion.

France had agreed an armistice with Germany the previous month. British forces had withdrawn from Dunkirk, but the British government had refused to seek peace. Germany controlled much of the western European coastline opposite Britain.

The directive did not order an immediate invasion. It instructed the armed forces to prepare one and set conditions that had to be met before a landing could take place.

The Royal Air Force had to be weakened until it could no longer offer effective opposition. Routes across the English Channel had to be cleared of British mines. German mines would restrict British naval movement, while heavy guns on the occupied coast would help protect the crossing.

The Royal Navy presented a larger problem. It was much stronger than the German navy and could send warships into the Channel if British forces remained able to act. The directive called for British naval forces to be occupied elsewhere or damaged before the invasion began.

German commanders disagreed about the operation. The army wanted landings across a wide stretch of the southern English coast. The navy favoured a narrower crossing that it had a better chance of protecting. Germany had few vessels designed to place troops, vehicles and supplies on defended beaches. Barges and other civilian craft had to be collected and converted.

Hitler ordered that preparations should be completed by the middle of August. This allowed little time to settle the disputes, assemble shipping, train troops and establish control of the Channel.

The air campaign over Britain became central to the plan. The Luftwaffe attacked shipping, airfields, radar stations and aircraft production in an attempt to reduce British air resistance. It inflicted serious losses but did not remove the Royal Air Force as an effective fighting force.

Without control of the air, German vessels crossing the Channel would remain exposed to British aircraft and warships. On 17 September, Hitler postponed Sea Lion indefinitely. The invasion was never attempted.