Analysis of information sources in references of the Wikipedia article "Battle of Singara (344)" in English language version.
Until the death of Constantine in 337, there was relative peace with the Romans, but the conversion of Armenia to Christianity and the Roman rulers' backing of Armenia caused Šāpur II to begin a campaign against them.
Šāpur II laid siege to Nisibis three times, and there was constant warfare, which did not go in favor of either side. The Roman defensive system of fortresses and limes hindered Šāpur's campaign in the region, but some forts, such as the town of Bezabde near Nisibis, fell to him.
The encroachment of the nomadic tribes in Central Asia forced Šāpur II to turn his attention to the East (Chronicle of Arbela, p. 85), and the war with Rome ended in stalemate by 350.