As one example, at the start of the war in Afghanistan, Marine scout snipers would operate in two-man teams. This was a Vietnam-era SOP that favored stealth over firepower--two men can't lay down much heat, but they don't need to if the enemy can't find and engage them. It's pretty easy to hide a couple guys in a jungle, so it worked well.
The problem with that doctrine in Afghanistan is that hiding even two men is difficult in arid environments. A team is far more likely to be seen regardless of its size and needs to be able to defend itself if compromised, which happens far more often in a desert. By 2010 the SOP was 8-man teams. At least one bloody incident was the cause of those numbers being bumped up.
There's a saying that rings true, "The military is always fighting the last war".
