Wholesale Hell Hath No Fury (Book 2 in new multiverse series) by David Weber

[Baen] It began with two men. They came from very different worlds¿entirely different universes, in fact¿and they met in a virgin forest on a duplicate planet Earth. Neither of them had expected it, both of them realized how important the first contact with any other trans-universal human civilization might be. But something went wrong. Neither side knows who shot first. But both the magic-using civilization of Arcana and steel-and-steam age Sharona, with its psionic Talents, think it was the other side. And it doesn't really matter, now, because the original incident has snowballed. Both sides have additional dead to mourn; both sides have additional wrongs to avenge. Both sides have additional military forces moving towards the front. War between the universes is the last thing responsible leaders on either side want. But the fury of their respective populations, xenophobic fear of the unknown, and cries for "justice" (or vengeance), are all driving both sides towards the brink. T