The battle was fought at the same time as the Siege of Calais, just before Operation Dynamo, the evacuation of the British Expeditionary Force (BEF) from Dunkirk.

General der Panzertruppe (Lieutenant-General) Heinz Guderian, the commander of XIX Corps, protested that he wanted to rush north up the Channel coast to capture Boulogne, Calais and Dunkirk.

An attack by part of XIX Corps was not ordered until 12:40 p.m. on 22 May, by which time the Allied troops at Boulogne had been reinforced from England by most of the 20th Guards Brigade.

The harbour is on a level area of ground on either side of the river ; well built-up and with steep roads uphill to the old town (Haute Ville or the Citadel).

The rolling hills make for hidden approaches to the port and offer commanding high ground to an attacker, particularly the Mont St. Lambert ridge.

Within a few days, the Germans achieved a breakthrough against the centre of the French front near Sedan and drove westwards down the valley of the River Somme.

[3][a] The Germans captured Abbeville at the mouth of the Somme on 21 May, cutting off the Allied troops in Northern France and Belgium from their bases further south.

[7] In the early hours of 21 May, Dutfoy ordered the naval garrison of 1,100 men to retire behind the thick medieval walls of the Haute Ville (Old Town or Citadel), east of the River Liane.

Dutfoy left for Dunkirk in the early hours and discipline broke down, a naval store was broken into and the looters drank the contents.

[8][b] Civilians still waiting for places on evacuation ships began to panic, until Capitaine de frégatte Poher, in charge of the sea front, threatened the crowd with a gun.

Some of Dutfoy's men contacted Vice-Admiral Marcel Leclerc, the deputy commander of Dunkirk, who ordered the remaining guns to be preserved for the defence of the town.

[9] Admiral Jean Abrial the French commander at Dunkirk issued an order, "You are to die at your posts one by one rather than give in..."[8] A detachment of Royal Marines arrived in Boulogne in the early morning of 21 May.

[12] The French 21st Infantry Division (Général de brigade [Brigadier-General] Pierre Louis Félix Lanquetot) was to hold a line between Samer and Desvres, about 10 mi (16 km) south of the town, where three battalions had already arrived.

[15] There were 1,500 men of No 5 Group Auxiliary Military Pioneer Corps (AMPC), a mixture of recalled reservists and part-trained troops working as labourers, in the town awaiting evacuation, under Lieutenant Colonel Donald Dean VC.

The southern column made contact first in the early afternoon of 22 May, against the headquarters company of the French 48th Infantry Regiment, the only troops of the 21st Division who were between the Germans and Boulogne.

In the early hours, the Germans attacked the Welsh Guards positions along the coast from the north-east as they began to envelop the town but were forced back each time.

[22] Fliegerkorps VIII (Generalmajor Wolfram Freiherr von Richthofen) sent Stukas to destroy the fortifications at Boulogne which was of great help to the attacking forces.

Fox-Pitt received orders from Vimy to hold Boulogne at all costs, as his radio contact with England had been lost earlier in the day.

A Luftwaffe raid was intercepted by Royal Air Force (RAF) Spitfires from 92 Squadron but the commanders of both British destroyers were killed by bomb splinters.

Frondeur was hit and disabled by Stuka dive bombers of I./Sturzkampfgeschwader 77, Orage was scuttled and the British destroyer Whitshed was damaged by a near-miss.

[30] Shortly before 6:00 p.m., Keith received orders for an immediate evacuation of the British and notification that five destroyers were either standing off Boulogne giving fire support or were en route.

Fox-Pitt decided to continue with the AMPC evacuation while the Guards conducted a fighting withdrawal to the harbour but communication with the British troops on the perimeter was only possible by dispatch rider.

Armed only with rifles, the pioneers had hoped to obstruct the Germans with makeshift roadblock barricades and claimed to have destroyed one tank by igniting petrol under it.

The quayside was deserted but when the captain called out by loud hailer many men appeared from hiding; the crew managed to squeeze them aboard.

[34] Lewis took over a large party of stragglers in the sheds at the quayside comprising guardsmen, 120 French infantry, 200 AMPC, 120 Royal Engineers and 150 civilian refugees; most of the Pioneers were unarmed.

When the sheds came under German fire, Lewis moved the group into the Gare Maritime (harbour railway station) and had sandbag barricades built.

[35] In the British Official History, Lionel Ellis wrote that the battle showed "how easily misunderstandings may arise between allies in such a confused situation".