"Britain certainly thought it had legitimate reasons for going in, and I think it did," she says."
You have pointed out that that reason was an Imperial one - part of one of your cut-'n-pastes.
It is not the job of historians - and historian - to pronounce whether that was legitimate - that is a moral, ethical or philosophical question, not a historical one.
Empire was about exploiting entire nations and using people and natural wealth for the benefit of the most powerful - go back in history as far as you want.
The British Empire collapsed under its own excesses a few decades after WW1   
If the war was one over colonies it was wrong - immoral - unethical.
The British people didn't benefit from any victory - we got nothing for the slaughter of our youth.
Things returned pretty much to the way they were at the beginning of the slaughter and before the twenties were out men were marching the streets demanding bread for their families.
The war never brought the improvements promised to those who fought - so which historian is qualified to say it was right - and why?
Jim Carroll