Edward I and Scotland
Edward's tombstone reads Edwardus Primus Scotorum Malleus hic est: "This is Edward I, Hammer of the Scots."
(What a sweet nickname.)
Edward and Scotland didn't start out as enemies. The troubles didn't begin until 1291, when the Scots, having suffered a series of really unlucky royal deaths, found themselves without a clear heir to the throne. There were thirteen claimants, but it really came down to John Balliol and Robert the Bruce. Edward picked Balliol, but afterwards demanded Balliol be subservient to him. Edward kept picking fights until war broke out, giving him an excuse to invade.
The whole situation is pithily explained over at Cracked (where they put Edward #1 on their list of "6 Historical Villains Who Were Actually OK Guys"):
Scotland: Help us, Edward Longshanks, you're our only hope!
Longshanks: Sure, I'll be glad to help. But first, I'll be needing Scotland.
Scotland: You'll be needing Scotland to do what?
Longshanks: To belong to England. I'll be needing you to give me Scotland.
Scotland: Oh. Er. Hm. OK, you can have our country, as long as you give it back when you're done.
Longshanks: ...Sure. I'll give it back. (rolls eyes)
Scotland: Huzzah! I don't see how this could possibly go wrong!
Seriously, Scotland? Had you even met England before?
Now, to be fair to Scotland, Edward had acted as peacemaker in other international pursuits. But not for countries of which he considered himself overlord. They really should have looked at his behavior towards Wales (that is, he refused to stop destroying stuff until he owned it) instead of that toward, say, France.
Here's a fun story about the siege of Dunbar (both of these quotes are from Michael Prestwich's Edward I):
[T]hose within the castle unfurled their banners, and directed the customary insults at the English, calling them 'tailed dogs', and threatening them with death and the amputation of their tails (it was a well-known myth in the middle ages that all Englishmen had tails).
Now, the reason William Wallace was so effective was because he used guerilla tactics--like all invadees facing a stronger opponent throughout the ages, the Scots did best when not forced into formal battle. Of course, the English did not take kindly to his methods or his success; after his capture, he was given a show trial:
The accusations against him reproduced some, but not all, of the English propaganda against a man who was feared and hated to a remarkable extent. He was accused of sparing none who spoke the English language, and of slaying infants, children, widows and nuns, but the curious charge that he had organized choirs of naked Englishmen and Englishwomen to sing for him, who were then tortured, was, understandably, not produced in court.
And this has been your fairly tangential monarch moment.
I'll admit, my heart wasn't totally in writing a second Edward I moment, because I am so! excited! for Edward II. So, as a preview, I bring you this segue--the account of the Edwards' most famous father-son moment. The backstory is that junior wanted his boyfriend, Piers Gaveston, to be given the territory of Pontieu. Instead of asking his scary dad himself, he got a bishop to do it:
The king was mightily enraged. "Who are you that dares to ask such a thing? As the Lord lives, you shall not escape my hands unless you can prove that you undertook this negotiation against your will, through fear of the prince. Now, however, you shall not leave until you see what he who sent you has to say." Having called for his son, the king said, "What negotiation have you promoted through this man?" His son replied, "That I might, with your acquiescence, give Ponthieu to my lord Piers de Gaveston." "You baseborn whoreson," shouted the king, "do you want to give away lands now, you who never gained any? As the Lord lives, if it were not for fear of breaking up the kingdom you should never enjoy your inheritance." And seizing a tuft of the prince's hair in each hand, he tore out as much as he could, until he was exhausted, when he threw him out.Walter of Guisborough, via The Oxford Book of Royal Anecdotes
Wow, good times.