Complex sentence: To effectively control the Alps as the shield of northern Italy, Rome had to control both flanks, and thus to extend its power to the Rhine and Danube, thereby also opening a direct route to Germania and all of Central Europe.
Split into simpler sentences: To effectively control the Alps as the shield of northern Italy, Rome needed to control both flanks of the mountain range. Thus it had to extend its power to the Rhine and Danube, thereby also opening a direct route to Germania and all of Central Europe.