Henry V landed near Harfleur on 14th Aug. and invested it. His small professional force suffered from dysentery, but the large French armies at Rouen, Harfleur and Caudebec sent no help to the town, which surrendered on 22 Sept. Henry paroled its leaders to meet him at Calais by 11 Nov. On 8 Oct. he set out with 5000 archers and 900 men-at-arms. Two large French armies shadowed him and prevented his crossing the Somme near its mouth on 13th. He now marched six days up river, found an obstructed but unguarded ford at Béthancourt and was across the river on 20th. He marched for Calais, the French moving parallel to but slightly ahead of him. Neither army knew exactly where the other was until Henry crossed the Ternoise on 24th and saw the enemy ahead. The French, with between 40,000 and 50,000 men, were determined to crush him. The route to Calais lay through a gap about a mile wide between two woods. The English were at the southern end of it: the French occupied the rest. The two armies slept the night facing each other. In the morning the English ran to the defile and set up oblique blocks of men-at-arms with the archers disposed so as to shoot across their front. The French had a mounted vanguard of 5,000-10,000; a main battle behind it of about 30,000 and a rear-guard of about 10,000. English morale was high because of the supine French conduct at Harfleur. The French commanders were ill-disciplined. Their attacks were badly coordinated and out of control. In half an hour their first two divisions were immobilised by overcrowding and muddy ground, and defenceless against the storm of arrows and then against their athletic opponents. The French lost about 6,000 men including the Ds. of Alençon, Bar, and Brabant, the Constable d'Albret and the Count of Nevers. The D. of Orleans was captured. The English lost no more than 300. Henry V reached Calais shortly afterwards.
Summary Article: Agincourt, Campaign and Battle of (25 October 1415)
From The Companion to British History, Routledge