Final answer:
The greatest threat to the Western Roman Empire was posed by the Huns who, due to their attacks and conquests, forced Germanic tribes to flee into Roman territories, causing significant destabilization. The Western Empire's long and chaotic border made it more vulnerable to invasions than the Eastern Empire.
Step-by-step explanation:
Who posed the greatest threat to the Western Empire? The Huns, expert horsemen, formed part of the nomadic tribes from central Asia that substantially contributed to the downfall of the Roman Empire during the fourth and fifth centuries CE. They attacked and conquered Germanic tribes such as the Goths, causing these tribes to flee into Roman territory, leading to significant destabilization.
The Western Roman Empire faced numerous challenges along its extended frontier, making it prone to political and social disruption. The long border stretched along the Rhine and Danube rivers, marking a tumultuous boundary more prone to invasions than that of the Eastern Empire. After the fall of Theodosius I in 395, these border vulnerabilities grew, with his sons, Arcadius and Honorius, inheriting the more secure East and the more vulnerable West, respectively.
While the Eastern Empire occasionally dealt with ongoing conflicts along the border with Persia, the fifth-century lull in Persian militarism spared them from facing the overwhelming odds that led to their Western counterparts' demise. Consequently, the Western Roman Empire, with its longer and more chaotic frontier, ultimately succumbed to external pressures, including the incursions of the Huns and other barbarian invaders.