To begin explaining the increasingly complex and fricative relationship between England and Scotland in the thirteenth century, it is worth exploring first some of the precedents that led to Edward I's claims of overlordship of Scotland.

First, the English claims of overlordship of Scotland, which they used to request/force submission of Scottish kings during the late twelfth century onwards, had little legal precedent. In 1173, William the Lion, king of Scots, joined the Young King Henry and the king of France in a rebellion against Henry II of England, known as the Young King's Rebellion of 1173-4. William was driven to join the rebellion because Henry II had taken away from him the earldom of Northumbria, which William held from King Henry as he inherited from his grandfather, David I of Scotland, who acquired the earldom by way of his marriage with Matilda of Senlis, Countess of Huntington and Northumbria (Matilda was the granddaughter of Siward of Northumbria). Thus, the king of Scots held two earldoms in England: Huntington (in Cambridgeshire) and Northumbria in the north of England. Henry's decision to take away the earldom from William had more to do with his legal and economic reforms to recover much of the royal lands that his predecessor, King Stephen of England, had given away to gather support for his kingship. William attempted to regain his English lands by ingratiating himself with Henry II, but the king refused to return the lands, which prompted William to join the rebellion. William raided Northumbria but was captured by English lords, who tied him to his horse and took him to Henry II. As a result, William had to sign the Treaty of Falaise in December 1174, which rendered the king of Scots as a client king subordinate to English kings and which claimed no legal precedent for this imposition. William was able to buy back his sovereignty from Richard I The Lionheart in 1189 for 10,000 marks that Richard used to go to Jerusalem on crusade. William's son, Alexander II (1214-1249) and grandson, Alexander III (1249-86), also submitted to English kings.

Edward I saw himself as heir to the Scottish crown because he was descended from Henry II, whose mother was Empress Matilda, daughter of Henry I and Queen Edith-Matilda, the latter who was, in turn, the oldest daughter of Malcolm III and Saint Margaret of Scotland. Edward requested Alexander to submit to him in England; in October 1278 in presence of Edward, Alexander refused to give homage to Edward for Scotland: “Nobody but God himself has the right to the homage for my realm of Scotland, and I hold it of nobody but God himself.” (See A.A.M. Duncan, Kingship of the Scots, 160-2. Needless to say, Anglo-Scottish relations, or at least regnal relations between Edward and Alexander, slowly eroded as Alexander sought to assert his regnal sovereignty from Edward's demands.

When Alexander died in 1286 without male issue, several men known as the Guardians of Scotland were selected to take political decisions while they decided how to solve the royal vacancy. Robert V Bruce and John Balliol were the closest descendants from the now-defunct Canmore dynasty; the increasing animosity between both factions threatened civil war in Scotland and because of this, it was the Guardians of Scotland who requested Edward I help in choosing the appropriate successor to Alexander III.

Further reading:

Reid, Norman H. (1990), Scotland in the Reign of Alexander III 1249-1286, John Donald, ISBN 0 85976 218 1 Barrow, G. W. S. Kingship and Unity: Scotland 1000-1306 (Edinburgh: Edinburgh University Press, 2003). Barrow, G.W.S. Robert Bruce and the Community of the Realm of Scotland (4th ed. 2005). Carpenter, David. The Struggle for Mastery: Britain 1066-1284 (Oxford, 2003). Duncan, A.A.M. The Kingship of the Scots, 842-1292: Succession and Independence (Edinburgh, 2002). Oram, Richard. Alexander II, King of Scots: 1214-1249 (Edinburgh: John Donald, 2005). McDonald, R. Andrew. Outlaws of Medieval Scotland: Challenges to the Canmore Kings, 1058-1266 (East Linton, Scotland: Tuckwell, 2003)

Right I've just done this in depth in history, Alexander was a pretty good king for numerous years and had 3 offsprings who died prior to his death, after he died the country went into turmoil after his grand daughter died resulting in no claim for the throne. 6 guardians led the country during the interrgum while 13 claimants rose up for the throne, most notably Robert de brus(grandson of the future king) and John Bailol, so civil war was on the horizon. The guardians sent a letter to Edward I asking him to quell the troubles and stop war breaking out, however he took the letters meaning as to crown the next king of Scotland and for Edward to be overlord. Thus began the great cause which lasted for 18 months and resulted in John Bailol being crowned king of Scotland, however Edward I was his overlord and Edward basically bullied him throughout his rule.