Think of Iraq and Afghanistan - their crystal ball is often as muddy as yours or mine.
While the 2010 Strategic Defence & Security Review (SDSR) did identify terrorism and international military crises as high level (tier 1) threats, there was no specific mention of Russia and so-called Islamic State (IS) did not exist in anything like its present form.
The 2015 SDSR will identify both IS and Russia as tier 1 threats, but they may not be the same threat five years hence.
They say that armies train to fight the last war they fought and military plans don't survive the first contact with the enemy.
"In 2010 it was rather assumed the armed forces would get out of Afghanistan and take a bit of a breather," says Michael Clarke, director of the defence think tank Rusi.
Instead "the world has shown us the armed forces won't be getting a holiday," he says.
Genuinely strategic?
First, the government should be commended on its commitment to carry out a defence review every five years. Before, defence reviews were done ad hoc.
But ministers will still have to overcome a high degree of scepticism as to whether this latest SDSR is genuinely "strategic" or if it matches Britain's global ambitions with the resources needed.
SDSR 2010 may have been strategic in name but it'll be remembered for the savage cuts that followed.
It wasn't just the scrapping of iconic names, it left gaping holes in Britain's defences - with no aircraft carriers for the Royal Navy and no maritime patrol aircraft to hunt down Russian submarines.
