Well that's just not true is it? The most ancient religious traditions that still exist in the world have not evolved into monotheism have they? And Judaism became monotheistic as a result of the strident militancy of the Yahwist cult that won out among the competing polytheistic and henotheistic idolatries and monolatries after the Babylonian exile - it did not evolve as an intellectually superior concept of deity, it was established as the dominant theology of Judaism by the, often forceful, often violent, subjugation of worshippers of "the Baals". This tension and the eventual outcome is evident in the books of the Law and the Prophets.
Correlation does not equal cause.
I see not connection between military conquest and the development of monotheism. There are plenty of polytheistic conquerors such as the Greeks and Romans. Rather, TIME passed, and with ideas being built upon (given the possibility due to written text) monotheism had the CHANCE to develop, and did so.
If there was any external event that sealed the deal, it was the Babylonian captivity.
The prophets had railed against the polytheism of the Israelites. They had adopted the idols of neighboring peoples in addition to God. God's such as Baal.
The earliest prophets seem to have urged Israelites to worship ONLY God among all the gods, meaning that the other gods were real in some sense. But at some point this changed to the other gods being FALSE gods. There is no greater example of this than Elijah's showdown with the priests of Baal, when Elijah called down fire from heaven, but the priests of Baal could not. But when did Israel accept this for good? In Babylon.
In its past, had Israel been conquered, it would have though to itself, "Hey, obviously the god of our conquerors is more powerful than Our God. We're gonna ditch Our God for theirs." (Good thing they were not conquered in those days.) But when they were conquered by Babylon, they remembered the words of the prophets, that other gods were false, and that the judgment for idolatry was coming. It shaped their assessment of their captivity. Instead of thinking that the Babylonian god was more powerful, they believed that they were being punished for their idolatry as had been prophesied. And THAT meant that the God of Abraham, Isaac, and Jacob was the ONLY God. Indeed, you have to admit, we Jews have never deviated into polytheism since Babylon.
But these prophetic visions of impending punishment for idolatry came from prophets who weren't building upon any conquering history. Their job was to correctly understand God's teaching and this was the ONLY thing they were concerned about.