Constantine was acclaimed Imperator by his legions on July 25th 306. Many successful generals were acclaimed by their troops. Not all became Emperor. For Constantine the key battle was Milvian Bridge in 312 against Maxentius.
The night before the battle Constantine had a vision from God. Or a meeting with the Christians of Rome. I explain what I think happened on this site in a post called Chi-Rho
In 315, again on July 25th, Constantine dedicated his triumphal arch in Rome in the plaza beside the Colosseum. By 330 AD Constantine had given up on Rome and founded his new Capital in the Greek town of Byzantium, renamed Constantinople.
The Arch of Constantine is brick faced concrete decorated with Marble. The sculptures and reliefs decorating the arch were stripped from earlier monuments created for Trajan, Hadrian and Marcus Aurelius. Nothing but the best for Constantine.
Rome: by Ezra Pound
O thou newcomer who seek’st Rome in Rome
and find’st in Rome no thing thou can’st call Roman;
arches worn old and palaces made common
Rome’s name alone within these walls keeps home.
Behold how pride and ruin can befall
one who hath set the whole world ’neath her laws,
all-conquering, now conquered, because
she is Time’s prey, and Time conquereth all.
Rome that art Rome’s one sole last monument,
Rome that alone hast conquered Rome the town,
Tiber alone, transient and seaward bent,
remains of Rome. O world, thou unconstant mime!
That which stands firm in thee Time batters down,
and that which fleeteth doth outrun swift Time.