Anglo-French forces trapped in Dunkirk perimeter, awaited rescue fleet from British Isles. May 27th, encircled Allied forces in Calais surrendered. May 28th, Belgium officially surrendered to Germany, its forces guarding Dunkirk left flank laid down their arms, creating an immediate defensive gap. Fortunately, this was filled at nightfall by British 3rd Infantry Division led by Major General Bernard Montgomery (of later WW2 fame). Bernard Montgomery - en.wikipedia.org/wiki/Bernard_Montgomery

Unexpectedly between May 24th-26th, Germans halted their armor drive upon Dunkirk, thereby allowing Allied forces to consolidate perimeter defense and accelerate evacuation. Debate on this panzer halt order ensued among historians, some say German Luftwaffe commander Hermann Goering wanted to demonstrate its true air power by destroying Dunkirk forces strictly from the air. Other say German Chancellor Adolf Hitler feared multitude of crisscrossing waterways and marshes would channel its armor forces into specific paths of advance, hence extremely vulnerable to Allied air strikes or artillery fire. Battle of Dunkirk - en.wikipedia.org/wiki/Battle_of_Dunkirk

Raw truth was that German Panzer forces consisted 30% of entire German Army, yet participated in 70% of its major battles since war commencement in 1939. These armor vehicles were breaking down from overheated engines, cracked chassis, transmission failures, etc. The panzer halt order gave these forces some downtime to conduct dire hardware repairs and replacements. Another delay factor was German encirclement of French 1st Army at Belgian city Lille (50 mile southeast of Dunkirk), invested by 7 divisions, 3 of which were Panzer Divisions. Lille - en.wikipedia.org/wiki/Lille

Rollover : Outlining map of three major Dunkirk evacuation routes (dotted lines), note interlaced coastal sandbars prevented direct evacuation route. Large ships originally came up to harbor pier for pickup, but after several hits and sinking by German Luftwaffe, this was impractical. Thus, smaller boats came up to the beach for pickup (about 250 small vessels were sunk during operation), while larger ships moored offshore for soldiers to wade across. Dunkirk - en.wikipedia.org/wiki/Dunkirk

French units conducted successful rear guard action in slowing down German advance, between May 27th and June 4th, 338,226 soldiers were rescued from Dunkirk beachhead, from this figure about 100,000 were French troops. Majority of soldiers from the 10 British divisions deployed to continent were rescued, loss of which would have seriously impaired both British morale and army personnel reconstitution. Urban warfare - en.wikipedia.org/wiki/Urban_combat