Once in a while during the campaign your Strategic Assets will come under attack these missions are optional. Successfully defend 5 of these in any number of ongoing campaigns. so it doesn't have to be in a single campaign you can have one save with 2 defended and another with 3 and it will pop.
these fights will consist of four waves with an elite enemy in the last. always alternating between two directions. where you have to beat a wave before the next arrives. These missions is how you easiest get a 5 star speed rating, under 5 minutes and potentially a 15 star rating at the same time.