Improvements in armor over the centuries led to the establishment of plate armor by the 14th century, which was further developed more rigorously in the 15th century. However, by the late 14th century and the early 15th century, socioeconomic calamities such as the Black Death, and political crises such as the Jacquerie peasant revolt, and especially the Armagnac-Burgundian Civil War combined with numerous English invasions, all led to French military power declining during the first two phases of the Hundred Years' War. New weapons, including artillery, and tactics seemingly made the knight more of a sitting target than an effective battle force, but the often-praised longbowmen had little to do with the English success. Poor coordination or rough and soft, muddy terrain led to a number of bungled French assaults. The slaughter of knights at the Battle of Agincourt best exemplified this carnage. The French were able to field a much larger army of men-at-arms than their English counterparts, who had many longbowmen. Despite this, the French suffered about 6,000 casualties compared to a few hundred for the English because the narrow terrain prevented the tactical envelopments envisioned in recently discovered French plans for the battle. The French suffered a similar defeat at the Battle of the Golden Spurs against Flemish militia in 1302. When knights were allowed to effectively deploy and attack their opponents in the open field, however, they could be more useful, as at Cassel in 1328 or, even more decisively, at Bouvines in 1214, and Patay in 1429.