Here is a brief list of William's service in war:
1277 & 1282 William served the king during the wars in North Wales
1286 William served with the king on the Continent
1287 Swansea Castle damaged by Rhys ap Meredith. Sacked and burned the town. Castle badly damaged.
1288 Blockade of [Newcastle] Emlyn started 1st Jan. 10th Jan William's siege engine arrived. 20th Jan castle fell with no man lost
1294 William served in west Wales where English settlements were being devastated by the Welsh.
1295/6 Served as 'keeper of the peace' in West Wales
1296 Served in Scotland
1296-98 Served in Flanders
April 1298 William was appointed a commissioner of array, to raise infantry from Gower to fight the Scots in Carlisle
June 1298 Fought at Falkirk – helped defeat William Wallace
July 1304 Reward for service in Scotland, the king granted a charter in October to William for royal jurisdiction
1310 William fought in Scotland
1314 William fought in Scotland at Bannockburn
1317 Another Scottish campaign