A war already under way

Dunkirk's story begins before the beaches, with the armies facing one another along the western frontier. Germany invaded Poland on 1 September 1939; Britain and France declared war two days later. The war was real from its first week. Poland was being destroyed, ships were being sunk, and airmen and sailors were already dying. What remained uncertain was when Germany would turn its main strength west, and what the Allies would do when it did.

British troops crossed to France during September under General Lord Gort. Over the winter the British Expeditionary Force trained, dug positions, improved communications and built up its equipment and supplies. The French Army carried the greater share of the continental commitment. A brief French advance into the Saar in September was followed by withdrawal. The long interval without a major western land offensive became the Phoney War: an unfortunate name if it suggests that nothing dangerous was happening anywhere.

Why a long war seemed possible

British and French leaders did not expect to defeat Germany in a single early offensive. They intended to hold, increase their military strength and use the resources of an alliance with powerful maritime and imperial connections. The memory of the First World War shaped that outlook. A hurried attack could produce terrible losses; a defended front, supported by artillery and preparation, appeared to offer a more dependable foundation.

This was a strategic wager on time. It required the armies to survive the opening German blow without losing their freedom to maneuver. A long war was possible only if the Allies could prevent a short campaign from breaking their continental position. That distinction matters: the men who prepared the defensive battle were not planning to surrender. They were relying on a particular way of absorbing the first attack.

What the Maginot Line could—and could not—do

The Maginot Line was a system of fortifications, weapons positions and supporting works along the eastern frontier, not a magic wall around France. Its strongest sectors made a direct attack expensive and allowed other troops to be deployed elsewhere. Fortifications could protect a sector; they could not decide where an enemy would seek the campaign's decisive opening.

The familiar story that France lost because it built forts and forgot that armies could move is too easy. The Allied plan positively required movement. Its best formations would advance into Belgium. The real question was whether that movement, the defenses farther south and the available reserves fitted together against the attack Germany actually launched. The answer would emerge at the Meuse, well south of the ground where the Allies expected their main contest.

The Dyle and the road toward Breda

Belgium was central to Allied thinking for sound geographical and political reasons. Allowing Germany to occupy it unopposed would expose northern France and place German aircraft and other forces closer to Britain. Moving forward also promised a defensive line farther from France's northern industrial region. Yet Belgian neutrality complicated preparation: an alliance could not simply treat neutral territory as its own deployment area before Germany attacked.

General Maurice Gamelin's plans evolved around an advance into Belgium. The Dyle plan took its name from the river along the intended defensive position. The Breda extension pushed a French commitment farther toward the southern Netherlands. Together these decisions sent important mobile forces toward the Allied left wing. They were meant to establish contact, strengthen the coalition's position and meet the expected German thrust before it reached France.

On a map, the attraction is clear. So is the risk. Forces advancing northeast could not instantly reverse direction and appear behind a breach far to the south. A reserve is useful partly because it has choices; once it is committed to a distant task, those choices narrow. The plan depended heavily on the first interpretation of German intentions being right.

Germany changes the question

German planning was neither unanimous nor effortless. The eventual Case Yellow gave the northern offensive a role beyond conquering territory: it would draw the strongest Allied forces forward while the decisive effort passed through the Ardennes and struck across the Meuse. Erich von Manstein was influential in advocating that southern emphasis, but a campaign of this scale was the work of multiple headquarters, commanders and formations, not a solitary flash of genius.

The proposal carried real danger. Large columns had to pass through restricted roads, cross rivers and keep fuel, ammunition and engineers moving behind them. A rapid advance could leave exposed flanks and a long supply route. Success depended on concentration, cooperation and maintaining the pace after a breach. Germany was taking a risk that its opponents would recognize the danger and respond too slowly to stop it.

The mistake was not a lack of courage

French and British troops possessed effective weapons, and many would fight with great determination. Germany's advantage cannot be reduced to a claim that every German tank was superior or every Allied soldier unwilling. Equipment mattered through organization: where it was concentrated, how it was supported and how quickly orders could become action. Radios, combined movement and decisions made closer to the fighting helped German formations exploit opportunities.

Before the first major attack, then, two different expectations were taking shape. The Allies intended to establish a defended front in Belgium and make Germany fight the battle they had prepared. Germany intended to make that preparation part of the trap. The next chapter follows the offensive of 10 May, when both plans began moving at once—and when the route toward Dunkirk was still invisible to most of the men who would eventually have to take it.