Possibly the most important social glue in medieval Europe was the oath. Everyone who could speak could take an oath. Though the courts might wrangle over how much an oath from a woman or a child could be legally upheld, everyone was expected to uphold their own oaths on a personal level.
This meant that the worst thing someone could do was renege on one's oath. It's no surprise that some of the longest and thorniest conflicts in the Middle Ages occurred over conflicting stories about oaths, particularly oaths of fealty.
For example, when the King of England, Edward I "Longshanks" invaded Scotland in 1296, he claimed that the King of Scotland, John Balliol, had reneged on an oath of fealty (this week's article) to Edward. Later, Edward brutally executed the leader of the Scottish resistance, William Wallace, as a rebel and an outlaw who had violated his own oath of fealty to Edward.
But of course, that is only one side of the story. Oaths were so important that they were hedged in by a variety of caveats. Some might dispute (as Wallace's supporters did) whether an oath was ever made, whether it was legitimate (for example, whether Edward had the authority to exact an oath of fealty from a foreign king) and whether it was made under duress (Robert the Bruce's swearing fealty to Edward and then breaking it). Not everyone agreed about what it took to make an oath. And only those with the authority to require an oath of fealty could compel one legitimately. Edward, being King of England, did not necessarily possess the authority to demand the fealty of the King of Scotland, let alone condemn those Scottish lords who resisted him as rebels and outlaws in their own country. Finally, an oath made under duress was often not considered legitimate. Medieval people didn't like bullies any more than modern people do.
History has not been kind to Edward's claims of legitimate rule in Scotland. Robert the Bruce, King of Scotland on Edward's death in 1307, got the last word by outliving Edward and beating his son, Edward II, at Bannockburn in 1314. The Scottish realm remained independent until 1603, when her king also became King of England, unifying the two realms under one crown. Not quite what Edward had had in mind when he envisioned Scotland under English rule.