The Protector's War — S.M. Stirling

It's been eight years since the Change rendered technology inoperable across the globe. Rising from the ashes of the computer and industrial ages is a brave new world. Survivors have banded together in tribal communities, committed to rebuilding society. In Oregon's Willamette Valley, former pilot Michael Havel's Bearkillers are warriors of renown. Their closest ally, the mystical Clan Mackenzie, is led by Wiccan folksinger Juniper Mackenzie. Their leadership has saved countless lives.

But not every leader has altruistic aspirations. Norman Arminger, medieval scholar, rules the Protectorate. He has enslaved civilians, built an army, and spread his forces from Portland through most of the western Washington State. Now he wants the Willamette Valley farmland, and he's willing to wage war to conquer it.

And unknown to both factions is the imminent arrival of a ship from Tasmania bearing British soldiers....