"The old custom of Marque and Reprisal had been revived with a vengeance for this war, and wherever Islander merchantmen met Tartessian, they fought."
-SM Stirling, On The Oceans Of Eternity (New York, 2000), Chapter Twenty-One, p. 429.
A letter of marque and reprisal authorizes a private military expedition. Stirling's time travelers apply the custom to combat at sea as Poul Anderson's space travelers apply it to combat in space. See here.
It is all one universe - or at least one multiverse.