Why did control of Spain, Gaul, and Syria give Augustus great military power?

In 27 BCE, Augustus was granted a 10-year tenure over a specific administrative region (provincia) that included Spain, Gaul, and Syria. This arrangement provided him with significant military power because these three regions contained the bulk of the Roman army.1

By controlling these provinces, Augustus maintained direct authority over the majority of the empire’s military forces, while the remaining provinces were governed by proconsuls appointed by the Senate. This strategic division ensured that the primary military strength of the empire remained under his personal command, effectively securing his position as the supreme leader while he maintained the outward appearance of restoring the republic.1