When you get advice from a barrister you will not be told that you will definitely win or lose. Instead, you are likely to be given advice framed in some other some form of words. Sometimes people don't quite understand what those words means. Let me explain.
Let's say that X comes to a barrister and asks whether he has a claim against Y. Broadly speaking, the barrister can come to one of 4 views of the matters: (a) X has a good strong case and it would be a big surprise if Y won; (b) X is more likely to win than to lose, but it would not be that surprising if Y won; (c) X is more likely to lose than win, but it would not be that surprising if X won; and (d) Y is probably going to win and it would be a big surprise if X won. (There is also (e) - X's case is so bad that the barrister cannot even advance it given his professional obligations - but take it from me, there is never a case at the other extreme, i.e. where you are 100% likely to win.) It's pretty hard to make finer distinctions than that 4-point scale. You might add in the truly 50-50 case or the almost-entirely hopeless argument, but you are unlikely to get to more than a 6 to 8-point scale.
Sometimes barristers are asked to put numbers on these prospects of success. You might see a 70% chance of success (rarely more than that) for the top category, 50-60% for the second and so on. More often, you see various well-worn phrases. It is important to understand that unless these phrases include "strong", "overwhelming" or something like that, they probably mean that the barrister thinks you are going to lose. So, for example, if you are told that your case is "reasonably arguable", that means it is in the third category: you will probably lose, but you've got a chance.
And that brings us to the UK and its wars. The Chilcott Inquiry tells us this about the advice that Lord Goldsmith gave the Government about the legality of the Iraq War:
"557. Lord Goldsmith wrote:
“In reaching my conclusions, I have taken account of the fact that on a number of
previous occasions, including in relation to Operation Desert Fox in December 1998
and Kosovo in 1999, UK forces have participated in military action on the basis of
advice from my predecessors that the legality of the action under international law
was no more than reasonably arguable.
“But a ‘reasonable case’ does not mean that if the matter ever came before a court
I would be confident that the court would agree with this view. I judge that, having
regard to the arguments on both sides, and considering the resolution as a whole in
the light of the statements made on adoption and subsequently, a court might well
conclude that OPs 4 and 12 do require a further Council decision in order to revive
the authorisation in resolution 678. But equally I consider that the counter view can
reasonably be maintained." (My emphasis.)
What Lord Goldsmith is saying here is that the practice of the UK Government has been to participate in military action where the legal case for doing so, in the view of the Government's legal advisers, is in the third category: i.e., where the chances are that if the matter were ever came to a final determination before a competent Court, the UK would probably lose the case. In short, it is good enough for the troops if the military action is probably illegal, but just not utterly and clearly illegal.
One moral from this is that you should be careful about expressing your opposition to a war based on the fact that it is 'illegal'. You may find that some other wars you did support were every bit as 'illegal' too.