The Hundred Years' War actually spanned 116 years (with frequent breaks in the middle) and was essentially England and France warring over control of France.
It all started in 1066 when William the Conqueror invaded Britain. He became the ruler of England but he also owned land in France's Acquitaine region (south west) thanks to the feudal system. So every following ruler of England inherited this piece of France and also the duty to support the King of France at times of war with either money or men.
Obviously the king of England did not like this idea, nor did the French nobles appreciate having an English king with a partial claim to their throne (although the English king(s) never seemed to take up this opportunity). One day the king of France (Phillip V) died without a male heir to the throne, this exposed to the French nobles how vulnerable to English take-over they were and so they decided to try to take the English king's lands off him. King Edward III responded by claiming ownership of the Whole of France (no point beating around the bush!!) and the wars started.
France was economically stronger than England and was able to mobilise more troops (and especially cavalry) than the English in the war but the English used the formidable longbow and seemed to have better tactics:
- Battle of Crecy (1346) - The English placed a mix of longbowmen and footsoldiers on a hill and picked off a charging french force consisting of mainly cavalry (horses running up hill against archers doesnt work!!!) the French lost 1500, the English had practically no deaths.
- Battle of Agincourt (1415) - King Henry V of England is walking his tired army back to Calais (and then England) after a successful capture of the french port of Harfleur when they are ambushed by a massive French army. The English army numbered about 6000 men, mostly lightly armoured archers (longbows naturally) while the French had 25,000 men - mainly heavily armoured cavalry and infantry. The English quickly took up position on top of a hill (sound familiar?) and the French cavalry tried to charge (you'd have thought they would have learnt the first time around) but it had been raining before the battle so the cavalry got stuck in mud at the bottom of the hill. After the archers had picked off most of the cavalry they charged down the hill to finish off the demoralised French infantry. Among the French casualties were no less than 500 noblemen (I don't have a figure for the total number of French deaths but you can take a good guess...) while the English lost less than 200 men.
This victory led to the English domination of northern France until the mid 15th century when the French got their act together and finally beat the English off their shores in 1558.