Why did the Roman Empire collapse-?

I think the most important reason why the Roman Empire collapsed was because it became too overstretched and large. That meant that the defence was great but too little. Their wasn't enough soldiers to defend all of the Roman Empire from great attack. It was too much like a Lobster. Once attackers got through the armoured shell there was nothing but undefended fleshy parts, and because the Empire was becoming to large, other problems because of this led from one to the other.

For example, More land to rule means more soldiers to guard it. More soldiers means more money and food you need to give them. More land ruled by Rome means closer enemies and more of them which can lead into soldiers deaths. Rome became so desperate they allowed 'barbarians' join the Roman army which led to them turning against them turning against Rome.