1. When something is very big, it cast a big shadow. Rome was huge and powerful so it's shadow is its extended influence. 2. The shadow of the past might refer to people who still remember WW2.

in your first sentence shadow means : 1- shadow of something /something : the strong (usually bad) influence of something /something. examples The new leader wants to escape from the shadow of his predecessor. These people have been living for years under the shadow of fear. source : Oxford Advanced Learner's Dictionary ................................................... in your second sentence shadow means : A vestige or inferior form example : shadows of their past achievements. source : http://www.thefreedictionary.com/shadow ( definition number 13 )

another usage: without a shadow of a doubt

I think in the first sentence shadow means stemming from evil characteristics or forces; wicked or dishonourable and in the second it means something existing in perception only cowboy...