Today's countries are incomparably more centralized than in the times of Rome.
When Georgia fired some missile on Russia (after weeks of harassment by Russian troops at the border), Putin knew about it within seconds. The same thing would have taken days if not weeks in Roman times.
And then, marching an army from a neighboring province again would have taken weeks, or even months if you needed to bring them from farther away. Today, your air force can arrive at any point on your (main) territory in a matter of hours, even for a country the size of Russia, and boots on the ground might take a day or two.
Also, everything about how war is fought and the impact of weapons has changed dramatically. The difference between the destructive power of the greatest (even non-nuclear) weapons that we have versus more common rifles or rockets is colossal. In Roman times, the difference between the best weapon and an improvised club was tiny by comparison.
When Georgia fired some missile on Russia (after weeks of harassment by Russian troops at the border), Putin knew about it within seconds. The same thing would have taken days if not weeks in Roman times.
And then, marching an army from a neighboring province again would have taken weeks, or even months if you needed to bring them from farther away. Today, your air force can arrive at any point on your (main) territory in a matter of hours, even for a country the size of Russia, and boots on the ground might take a day or two.
Also, everything about how war is fought and the impact of weapons has changed dramatically. The difference between the destructive power of the greatest (even non-nuclear) weapons that we have versus more common rifles or rockets is colossal. In Roman times, the difference between the best weapon and an improvised club was tiny by comparison.