At the age of 21 Walter fought against the English at the
Battle of Bannockburn in 1314 where according to some sources, together with Douglas he commanded the left wing of the Scots' army, but according to other sources was (due to his youth and inexperience) merely the nominal leader of one of the four (or three) Scottish
schiltrons, the effective leader being his cousin
James Douglas, Lord of Douglas. For his services at Bannockburn, Walter was appointed Warden of the Western Marches and was rewarded with a grant of the lands of
Largs, which had been forfeited by King
John Balliol. In 1316 Stewart donated those lands to
Paisley Abbey. Following the liberation of King
Robert the Bruce's wife,
Elizabeth de Burgh, and daughter,
Marjorie, from their long captivity in England in October 1314, Walter the High Steward was sent to receive them at the
Anglo-Scottish Border and conduct them back to the Scottish royal court. During the absence of King Robert the Bruce in
Ireland, Walter the High Steward and Sir James Douglas managed government affairs and spent much time defending the
Scottish Borders. Upon the capture of
Berwick-upon-Tweed from the English in 1318 he took command of the town which subsequently on 24 July 1319 was besieged by King
Edward II of England. Several of the
siege engines were destroyed by the Scots' garrison whereupon Walter the Steward suddenly rushed in force from the walled town to drive off the enemy. In 1322, with Douglas and
Thomas Randolph, he made an attempt to surprise the English king at
Byland Abbey, near
Malton in
Yorkshire, but Edward escaped, pursued towards
York by Walter the Steward and 500 horsemen. ==Marriages and children==