Edward's vigorous reign was characterized by constant warfare. Trouble with Llywelyn ap Gruffydd led to his successful conquest (1277-82) of Wales beyond the Welsh Marches, and in 1284 he extended the English administration to Wales. In France from 1286 to 1289 he improved the administration of Gascony.

After the death in 1290 of Margaret Maid of Norway, Edward asserted his claim to overlordship of Scotland, but John de Baliol (1249-1315), his choice for the throne, soon entered an alliance with Philip IV of France, with whom Edward was already on bad terms. Edward's long struggle to conquer Scotland began in 1296. His first campaign was successful; he deposed Baliol and humiliated Scotland by removing the Coronation Stone (see under coronation) from Scone to Westminster. But while he was heading an expedition against France in 1297 the Scots found a new leader in Sir William Wallace, who defeated the English at Stirling Bridge.

Edward immediately concluded a truce with Philip IV, and the English claims to Gascony were finally settled favorably in the treaty of 1303. In the meantime Edward invaded Scotland again and won a brilliant but inconclusive victory at Falkirk (1298). Campaigns in the following years led to Wallace's defeat (1305) and execution, but a new leader, Robert I, arose as king of a still defiant Scotland. Edward commenced an expedition against him in 1307 but died before reaching the border.

Even more important than Edward's military exploits were the legal and constitutional developments of his reign; Edward has been called the English Justinian. He asserted the judicial supremacy of the crown by his quo warranto proceedings (inquiries to determine "by what warrant" private jurisdictions were held), which culminated in the statutes of Gloucester (1278) and of Quo Warranto (1290). By his law of 1285, Circumspecte agatis, he forced church courts to confine themselves to ecclesiastical cases. His three statutes of Westminster (1275, 1285, 1290; see Westminster, Statutes of) formulated the advances of a century of common law and supplemented them.

By his Statute of Mortmain (1279), Edward prohibited grants of land to the church without the king's permission. In turn the English clergy, backed by Pope Boniface VIII's bull Clericis laicos (1296), refused in 1297 to contribute to Edward's campaign against the French until the king boldly denied protection to them and their goods and even threatened to confiscate all church property. This action was mainly prompted by his need for funds, as was his expulsion (1290) of the Jews from England (which enabled him to seize their property). His expensive wars also necessitated the frequent summoning of Parliament to grant taxes. The so-called Model Parliament of 1295 included representatives of the shires, boroughs, and lesser clergy, but the composition of Edward's parliaments varied.

The increasing resistance of the country to heavy taxation and the refusal of many barons to fight in France in 1297 forced Edward to issue a confirmation of the charters of liberties, including the Magna Carta and those signed by Henry III. The king also promised that he would collect the nonfeudal forms of taxation only with the consent of Parliament. He did not keep this promise, however, and the last years of his reign were marked by increasing baronial opposition to the crown. This opposition and the war with Scotland proved to be a disastrous legacy for his son and successor, Edward II.