Compare competing historical theories -- moral decline, multi-causal internal weakness, and outside migration -- for why the Western Roman Empire declined, evaluating each against evidence.
Historians have debated why the Western Roman Empire declined for longer than the empire itself lasted in some tellings, and studying that debate is itself a historical-thinking skill, not just a rehash of facts. In the 1700s, the historian Edward Gibbon argued in his massive The History of the Decline and Fall of the Roman Empire that the spread of Christianity and a broader loss of traditional civic virtue softened Rome from within -- an influential theory for centuries, though most historians today see it as, at most, one factor among several rather than the main cause.
Modern historians tend to point to a tangle of causes acting together: the political chaos of the third century CE, when dozens of emperors ruled only briefly before assassination; economic trouble from inflation and heavy taxation; a military increasingly dependent on non-Roman soldiers; and growing pressure from migrating peoples such as the Goths, Vandals, and Huns. Historians still weigh these causes differently -- some emphasize outside migration pressure as decisive, others emphasize internal political and economic rot as the deeper cause, and some argue Rome did not so much "fall" as gradually transform into medieval Europe over generations. The traditional end date, 476 CE, when the Germanic general Odoacer removed the last western emperor, is useful precisely because a single, centuries-long process needs at least one clear marker -- but a good historian asks whether any single year can really capture a slow transformation.
Mastery looks like: They can describe at least two different historical theories for Rome's decline and explain that historians still debate which factors mattered most.
Common stumbles: Treating one single cause (such as Christianity, or "barbarian invasions") as the complete, settled explanation for Rome's decline.