After the acclaimation Iulianus decided to send messengers to inform Constantius II that he too was now an Augustus.

When the messengers got to Caesaria in Cappadocia and gave the message to the Augustus he flew into a rage.
Yet he could not march against Iulianus as his own military forces were occupied fighting the Persians.
So he encited the Germanic tribes along the Rhine to invade the domain of Iulianus.

And so for the rest of the year 360AD Iulianus was fighting of the Franks and other Germans.
In October he headed south for Vienne to celebrate fis acclaimation of Caesar five years previously wearing more ostentatios apparell, perhaps he was not sure if his new title of Augustus would last long...

By the spring of 361AD there was more fighting along the Rhine, which ended only after the chief of the Alemanni was invited to a banquet by the local Roman commader and arrested soon after.
As for some kind of accomadation being reached between Iulianus and Constantius II that was now impossible.
There was a truce in the Persian war and this gave Constantius the time to prepare his troops to march against Iulianus.
Iulianus was still not sure whether to wait in his domain for the invasion or go out against him.
He prayed to the godess Bellona sacrificing a bull and believed to have been given a sign telling him to march out against Constantius assembling his troops at Vienne.

Yet he could not leave the newly pacified Rhine frontier undefended, this was garrisoned by those troops who refused to march east.
It was an army of 25,000 that he ended up having, this was far less than the huge army Constantius was likely to field against him.

So Iulianus devised a strategem, dividing his army into three.
10,000 were to march across the Alps into Italy and then head for Illyria.
Another 10,000 or so were to head for Noricum to also head for Illyria.
The remainder were led by Iulianus in person and taken through the reaches of the upper Danube to go onto boats and head downstream.
The entire army was to meet at Sirmium on the Sava river.