Where were Caesar's legions when the Civil War began?

When Julius Caesar crossed the Rubicon during the evening of January 10th, 49BC, he had nine legions spread throughout the two Gallic provinces and was in the process of raising another three. Of these there were his four famous Spanish legions - the 7th, 8th, 9th and 10th, plus his newer Gallic Legions the 6th (the Italian Draft from 52BC), 11th, 12th, 13th and 14th. Being January, the legions were in their winter camps, the 7th, 10th, 12th & 14th were in the Rhone Valley city of Decetia under the command of Caesar's Senior Legate, Gaius Fabius. The 6th, 8th, 9th &11th were in the lands of the Belgic Remi at Andematunnum under the other Senior Legate, Gaius Trebonius. Julius Caesar was in Ravenna, his Cisalpine Gaul base with just the 13th Legion.

So when Caesar is crossing the Rubicon he had just the 13th - but he was planning on backup. Trebonius had sent the 8th legion through St Bernards Pass (in the middle of winter) and it was just sixteen days behind the 13th, while Fabius had sent the 12th across the lower Alps. After subduing Northern Italy Caesar chased his conservative enemies to Brundisium with the 8th, 12th & 13th, plus his new recruits from Cisalpine Gaul and a few thousand Italian conscripts he'd shaped into the 16th, 17th and 18th Legions along the way.

Meanwhile back in Gaul - Trebonius was heading south during January with the 6th, 9th and 11th to wait at Vienna in the southern Rhone Valley to see if Caesar would need more help in Italy or if  Fabius - who was moving towards the Spanish border with the 7th, 10th and 14th - ran into Pompey's Spanish Legions. At this stage the independent Greek city state of Massilia wasn't an issue. Things only changed there when Caesar was taking the 12th, 13th and 18th from Italy back across Gaul to join Fabius and Trebonius for a war on Spain in April.

He more or less came across Massilia in the hands of the enemy during what he intended to be peace negotiations. Luckily for him he had three legions with him (the 12th, 13th and 18th) but he wanted to go to Spain and have someone else take command of a simple urban siege...and since it was easier to transfer a Senior Legate than an army, he brought Trebonius over to Massilia. So by April 49BC the veteran Gallic legions, the 12th & 13th plus the newbie Italio/Gallic 18th Legion were besieging Massilia, while the 6th, 7th, 9th, 10th, 11th & 14th were the six legions in Spain. For more on the Roman world read 'Vagabond' - available from Amazon, just follow the links.