Final answer:
The British army's failure at Yorktown was primarily due to the help from France, as French naval and land forces crucially supported the American troops, leading to Cornwallis's surrender.
Step-by-step explanation:
The decisive defeat of the British army at Yorktown can be largely attributed to the help from France, both on land and sea. During the Yorktown campaign, the presence of a large French fleet under Admiral de Grasse in the Chesapeake Bay prevented the British from using their naval superiority to evacuate or reinforce General Cornwallis's army. Additionally, the combined French and American forces, totaling over 16,000 men under the leadership of General Washington and French commanders like the Marquis de Lafayette and the Comte de Rochambeau, effectively surrounded Cornwallis, leaving him with no option but to surrender.