Having read Christopher Clark's The Sleepwalkers a couple of years ago, with its magisterial account of the complexities of the origins of the First World War, the parallels did appear clear.
A bit of Googling - Google 'Sleepwalkers Brexit Ian Jack' - found Ian Jack's perceptive piece from last November.
The parallels lie in the complexity and unpredictability of the situation. Multiple stakeholders with differing and shifting perspectives in each of the countries involved - just think of our political parties. Thus the potential for a causal avalanche in any of several directions. It is the chaotic nature of the situation which could lead to an outcome which nobody wants - just like the First World War.