But was WW1 quite the mistake it often seems? Here's what I wrote in response to a similar point on the ever-thought-provoking Kate's Home Blog.
Let's just look at what would have happened had the conflict still taken place but with Britain standing on the sidelines. The war would effectively then have been between Imperial Germany/Austria-Hungary on the one side and Russia/France on the other.
In all likelihood, Imperial Germany would have won, which would have meant it controlled virtually the whole of continental Europe stretching from the Atlantic to the Arals. With Russia defeated, there would probably still have been a Russian revolution, but the resulting Russian republic would have been very much a client state of the dominant European superpower, Germany.
The really intriguing counterfactual point about a German victory in WW1 is that the Hohenzollern dynasty would have continued, and Adolf Hitler would in all likelihood have remained an impoverished painter.
So by entering the war, winning it, and giving the Germans a thirst for revenge, we ended up with Hitler. By not entering, we would have ended up with a German superpower controlling the whole of Europe. Take your pick...
I think what this demonstrates is that quantifying whether something is a "mistake" or not is very hard. History tends to have a Yin and Yang about it, and sometimes apparently "bad" things have unexpectedly "good" consequences.
A Christian would say that God ultimately redeems everything, or in the words of an old worship song "he turns our weaknesses into his opportunities."
It's not in fact very Christian, but I would probably have to say that the biggest mistake in recent history was the Clinton administration's failure to take out Osama bin Laden when they had the chance in the mid-90s. Sadly, they were distracted by Kenneth Starr and the Lewinsky scandal.
Closer to home, here are half a dozen things which I wish recent British governments had done differently, the consequences of which have been pretty baleful for all concerned, and which continue to be felt today.
1. Introduced the Barnett Formula (Labour, 1978)
2. Paved the way for Robert Mugabe to take over Zimbabwe (Tories, 1980)
3. Allowed Rupert Murdoch to buy The Times and the Sunday Times (Tories, 1981)
4. Privatised the railways (Tories, 1996)
5. Chosen Rowan Williams as Archbishop of Canterbury ahead of Michael Nazir-Ali (Labour, 2002)*
6. Joined the invasion of Iraq (Labour, 2003)
* For the benefit of those who have asked me for my take on the Archbish.