 In the last years of his life Constantine made plans for a campaign against Persia . In a letter written to the king of Persia , Shapur , Constantine had asserted his patronage over Persia 's Christian subjects and urged Shapur to treat them well . The letter is <unk> . In response to border raids , Constantine sent Constantius to guard the eastern frontier in 335 . In 336 , prince Narseh invaded Armenia ( a Christian kingdom since 301 ) and installed a Persian client on the throne . Constantine then resolved to campaign against Persia himself . He treated the war as a Christian crusade , calling for bishops to accompany the army and commissioning a tent in the shape of a church to follow him everywhere . Constantine planned to be baptized in the Jordan River before crossing into Persia . Persian diplomats came to Constantinople over the winter of 336 – 337 , seeking peace , but Constantine turned them away . The campaign was called off , however , when Constantine became sick in the spring of 337 . 
