The Crimson Campaign / Brian McClellan.

Enhanced descriptions from Syndetics:

"Just plain awesome" -- Brandon Sanderson
A staggering defeat on the border of Adro and Kez leaves Tamas and his powder mages in grave danger and cut off from all aid. Meanwhile, deadly political games threaten his allies at home.

When invasion looms. . .

Tamas's invasion of Kez ends in disaster when a Kez counter-offensive leaves him cut off behind enemy lines with only a fraction of his army, no supplies, and no hope of reinforcements. Drastically outnumbered and pursued by the enemy's best, he must lead his men on a reckless march through northern Kez to safety, and back over the mountains so that he can defend his country from an angry god, Kresimir.

But the threats are closer to home. . .

In Adro, Inspector Adamat wants only to rescue his wife. To do so he must track down and confront the evil Lord Vetas. He has questions for Vetas concerning his enigmatic master, but the answers will lead Adamat on a darker journey.

Who will lead the charge?

Tamas's generals bicker among themselves, the brigades lose ground every day beneath the Kez onslaught, and Kresimir wants the head of the man who shot him in the eye. With Tamas and his powder cabal presumed dead, Taniel Two-shot finds himself as the last line of defense against Kresimir's advancing army.

The Crimson Campaign continues the most acclaimed and action-packed new epic fantasy series in years, following on from Promise of Blood - an adrenaline-fuelled debut of flintlock mages, civil war, and cold-blooded murder in a world where gods walk the earth.