Haha, so that's quite a simple question with a very complicated answer.

To begin there is a bit of background history that you probably should know, (sorry, not sorry).

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In 1283, Edward had finished his conquest of Wales and began to look to other places to expand his power. Edward was a military man and an excellent tactician, so he spent most of his time as king preparing for, or fighting wars. Scotland during this time (1280s) was experiencing a 'golden age' under Alexander III, who was modernising the country, and Scottish-English relations were at an all-time high with Edward I's sister (Margaret of England) being married to Alexander III. However, things drastically changed when Alexander III died leaving only a young granddaughter to inherit the throne (Margaret, the Maid of Norway). Quickly the Scottish nobles arranged a marriage between Edward I's young son (Edward II) and the future queen of Scotland (The Treaty of Brigham/Salisbury). This pleased Edward I, giving him the sudden realisation Scotland could be an addition to his growing empire.

But, everything went sideways because Margret, the Maid of Norway also died before she could take the throne (lol). This is what gave Edward I his drive to invade Scotland later on, he had already come so close.

(Please note that if the marriage went through Scotland and England would still be separate countries, however having an English monarch's bloodline on the throne of Scotland is the basically next best thing)

This once again left Scotland without a king, leading into a period called "The Great Cause", where a man know as "John Balliol" was chosen to rule.

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Okay, that's where our story begins - so let's get on to the more relevant history:

During 'The Great Cause' all the claimants to the Scottish throne swore allegiance to Edward I, as he was chosen to choose the next Scottish king (because he's a law expert, all-round nice guy and nothing bad could ever come from him). This meant that John Balliol was basically left as Edward's puppet, and was forced to obey Edward I, who developed a habit of overruling him. This started to rub the Scots (and Balliol) up the wrong way, who finally broke as Edward declared war on France (1294) and demanded the Scots fought for him. This infringed on Scottish sovereignty and forced the Scots to sign an alliance with the French, and surprise, this leads to Edward's swift invasion of Scotland and The First Scottish War of Independence.

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Summary (for people who don't like reading):

- Edward I wanted to expand his territory.

- He realised Scotland would be a logical next step.

- He was chosen to choose a Scottish king.

- He abused this position to make the next king a puppet. (legally).

- He escalated Scottish-English tensions to get an opportunity for war.

- He then won his war.

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Motivation: Expansionism and power seeking.

Justification: He could. And everything he did was completely legal and 'above the table'. The Scots were generally unaware of his intentions right until the end and walked right into it.

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