Here's an example, from Herodotus' Histories:
"Herein, too, God showed forth most evidently, how much better a thing for man death is than life."
And this, from Xenophon's Hellenica:
"Only by God's help let us show these barbarians that we do not need to worship them, in order to punish our foes."
Is "God" a reference to Zeus in these contexts? Or did they have some concept of monotheism?
This is a quirk of older translations that I've written about before. In most cases the Greek does not actually refer to a single capital-G God but to ho theos, "the god." This is a deliberately vague reference to divine power. Greek authors used this because they were conscious of the limitations of their knowledge of the divine, and generally did not presume to know which god had acted or why. The phrase "the god" allowed them to give the cause of something without falsely ascribing agency to any particular god.
The lines you cite are good examples. Herodotos 1.31.2:
"The god (ho theos) made clear that for human beings it is a better thing to die than to live."
Xenophon, Hellenika 1.6.11:
"Let us then, with the help of the gods (sun tois theois), show the barbarians that even without paying court to them we can punish our enemies."