Out of the Iron Womb by Poul Anderson

You can't just lie here and let him come kill you.

There was a picture behind his eyes; he didn't know if it was a dream or a long buried memory. He stood under an aspen which quivered and rustled as if it laughed to itself softly, softly, when the wind embraced it. And the wind was blowing up a red granite slope, wild and salt from the Sound, and there were towering clouds lifting over Denmark to the west. The sunlight rained and streamed through aspen leaves, broken, shaken, falling in spatters against the earth, and he, Bo Jonsson, laughed with the wind and the tree and the far watery glitter of the Sound.