You can proclaim it, but that doesn't make it true. The late Roman Empire had it as a motto; they fell eventually. Germans in World War II had a motto "Gott mit uns" but they lost the war.
It goes back to Isaiah 7:14, quoted in Matthew 1: 23:
'Behold the virgin shall conceive and bear a son, and his name shall be called Immanuel, which being interpreted is God with us'
So Christians can claim "God is with us" as they fight each other.