Of the many links generated by the Crown, military connectivity was one of the most potent. As they forged England into a ‘war state’, the three Edwards with their bellicose ambitions mobilised the people and resources of their realm on a scale that dwarfed anything attempted by their predecessors. Inevitably, Cornwall was drawn into the war effort. Between the battles of Falkirk in 1298 and Agincourt in 1415 there were no fewer than 452 instances of named Cornish men-at-arms and mounted archers fighting in major royal expeditions. Yet there were wide variations in the degree of participation. While no fewer than twenty-seven Cornishmen served at Crecy-Calais, for example, only two enlisted in 1373 to go with John of Gaunt on his ‘great march’.

The changing demands of war greatly influenced the nature and scale of Cornish military service. While the large Falkirk and Bannockburn campaigns drew about eighteen gentlemen to Scotland, the smaller armies operating north of the border in the 1330s were more poorly attended. The Hundred Years’ War was to result in a dramatic expansion of the county's service. In 1340 some eight Cornishmen served at Sluys-Tournai, but just two years later twice that number fought in Brittany, a campaign for which the peninsula was more conveniently sited. With some twelve Cornishmen contesting the field at Poitiers, the county was well placed for expeditions to Aquitaine too. From 1369 royal demands for troops grew more regular, as ‘the Edwardian War was intermittent; the Caroline War (1369–89) unremitting’. In a war which favoured the county's strong military-maritime tradition, England and France increasingly fought for command of the sea routes. It is illustrative of this that the earl of Arundel's ‘cruising war-fleets’ of 1387 and 1388 contained as many as fifty-six Cornish men-at-arms and archers. Since England and France increasingly drew other kingdoms into this phase of the war, in 1386 some nine Cornishmen campaigned with John of Gaunt in his bid for the Castilian throne. In these years Princess Joan can even be found paying her steward an addition £13 6s. 8d. for raising recruits to fight in France and Spain.