ZOUR MAGHAR, Syria — On the eastern bank of the Euphrates River, Kurdish militiamen aligned with American troops burrow into sandbagged positions and eye their foes across the water.

On the other side, Arab rebels backed by Turkey shoot at anyone who nears the river.

For millenniums, the Euphrates has given farmers in the village of Zour Maghar water to irrigate fields of wheat, eggplant and sunflowers. Generations of families have sprawled on its banks for picnics, the older children teaching the younger to swim.

But after seven years of war, the river that has fed life in Syria’s parched east has become a hostile front, separating warring sides as it travels north to south. Deprived of its water, families have fled Zour Maghar, abandoning their mud-brick homes and leaving their fields idle.