In 1997, the American journalist Sebastian Junger published a book entitled The Perfect Storm. It was an account of a major storm in America in 1991, which killed 13 people and caused damage valued at $200m.
When he was researching it, he spoke to Bob Case, a meteorologist in the National Weather Service at the time of the storm.
Case described it as the confluence, or coming together, of three different weather-related phenomena that combined to create what Case said was the "perfect situation" to generate such a storm. That was why Junger called it "the perfect storm".
It is a concept that can be helpful when we think about the Troubles in Northern Ireland and a terrorist campaign that lasted for so many years.
Last month, the respected journalist Peter Taylor posed the question: "Who won the war?" It is an interesting and important question and it led me to think of an equally interesting and important question: "Who started the war?"
After 1921, there were sporadic outbreaks of republican violence in Northern Ireland and there were riots in Belfast and other places in the 1920s and 1930s.
But the IRA campaign during the Second World War, which was linked to the IRA collusion with the Nazis, was shambolic and the IRA border campaign that started in 1956 staggered on at a low level before finally petering out in 1962.
There was never a terrorist campaign that lasted so long, with such intensity and involved so much murder and destruction as what we saw in the years after 1969.
Why, then, did the previous IRA campaign end in abysmal failure in 1962 and yet, within just seven years, Ulster had exploded in the summer of 1969? What was it that happened?
I would suggest that, in the 1960s, we had the "perfect storm" - a situation where a whole series of factors came together in a way that they never had before.
In their preparation for the 1956-1962 campaign, the IRA resorted to a series of arms raids to get guns and ammunition, but they were poorly armed.
This time, the role of Libya in arming the Provisional IRA and the support of Irish-Americans were hugely important.
Even before that, there was the role of senior Fianna Fail politicians, including government ministers, in the "arms crisis".
The "arms trial" was something of a farce and Captain James Kelly was hung out to dry, but since then more of that story has emerged. There were also official reports of the brother of a Fianna Fail minister being in London in the company of a member of Saor Eire, a small Trotskyite terror group, which supplied arms to what became the Provisional IRA. What was the nature of that co-operation, or collusion?
During the previous IRA campaign, a Fianna Fail government introduced internment, but this time, instead of interning IRA terrorists, Fianna Fail ministers were arming them.
Then, too, what was the real story of the Northern Ireland Civil Rights Association and what part did the Connolly Association, the IRA, the Communist Party of Ireland and the Communist Party of Great Britain play in creating that organisation?
And what role did the Communist Party of Ireland play in facilitating links between the Official IRA and the Soviet regime in Russia?
We already know that one of the leaders of the CPI passed on the request from the Official IRA for arms from Russia.
In the previous IRA campaign, there was a conscious decision to avoid Belfast, because it was believed that the situation in the city was just too combustible.
But, in 1969, "civil rights" leaders in Londonderry, including members of the IRA, sent out the message to Belfast to "take the heat off Derry".
These are just a few of the factors that came together for the first time and the result was a "perfect storm".
Ulster exploded - and the explosion continued for another 30 years.
Nelson McCausland MLA is chair of the Assembly's culture, arts and leisure committee