Final answer:
The most crushing defeat of the Romans was in the Teutoburg Forest in 9 CE, where several legions were decimated. Other significant challenges to Roman military power included the extensive Gallic Wars, Hannibal's successes in the Punic Wars, and the Gothic attack on Rome that breached its walls for the first time in seven centuries.
Step-by-step explanation:
The most crushing defeat that the Romans suffered was in the Teutoburg Forest in 9 CE. This catastrophic event saw the annihilation of several Roman legions and signified the end of Roman territorial expansion across the Rhine. The battle, which occurred over three days, is particularly noteworthy for its detailed account of a Roman army's complete collapse, a level of detail seldom available for other battles of the period.
Roman conflicts with other regions also demonstrate the extent of Roman warfare and its consequences. The Gallic Wars led by Julius Caesar in the 50s BCE caused immense devastation in Gaul, resulting in the death or enslavement of millions. Rome's military might and consequent resilience are further illustrated by their continual conflicts, including the Gothic attack that breached the walls of Rome, which hadn't been done by a foreign enemy in around 700 years, and their wars with King Mithradates of Pontus, albeit at a great cost.
In another notable defeat, Roman forces were bested multiple times by the Carthaginian general Hannibal, most prominently at the Battle of Cannae in 216 BCE. Despite these setbacks, including issues of logistics and internal unity in later centuries, Rome's military adapted over time, even adopting Persian technologies and military tactics.