Cum clause:
When- temporal
Since/ although

When this plan was announced to Caesar, that the Helvetti were trying to make their way though our province, he hurried as fast as he could from the city [Rome] across Gaul and ordered as many troops as he could in as great a number as he could from all over the province (and in all of Gaul there were 6,000 men).  They were to meet at a bridge in Geneva.