Rome did not invent most of the weapons it carried. The gladius came from Iberia, the scutum from Italic neighbors, the pilum from the Samnites. What Rome did, with terrible efficiency, was test, refine, standardize, and mass-produce. The result was a military machine that lasted, in one form or another, for over a millennium.
A republic at war
The early Roman Republic fought constantly. From the wars with neighboring Italic tribes to the three Punic Wars against Carthage, the Roman citizen-soldier system was hardened in conflict. Each adult male citizen was expected to serve, providing his own armor and weapons according to his wealth class. The wealthier the citizen, the heavier his equipment — and the further forward his place in the battle line.
The professional army
The Marian reforms of 107 BC changed everything. Gaius Marius opened the legions to landless citizens, paid them a salary, and standardized their equipment. The result was the imperial Roman army: a professional standing force, equipped from state arsenals to identical specifications, drilled to fight in tactical units (centuries, cohorts, legions) that moved as a single body.
The legion in the field
A late Republican or early Imperial legion of roughly 5,000 men carried, for each soldier: a gladius, a pugio, two pila, a scutum, body armor, helmet, marching kit, and supplies for several days. They built fortified camps every night they marched. They could construct roads, bridges, siege engines, and walls. The legion was as much an engineering corps as it was a fighting force.
The end of the system
By the 3rd century AD, the classic legion had broken down. Larger barbarian armies on the frontiers, increasing reliance on cavalry, and economic strain pushed the army toward looser formations and lighter equipment. The gladius gave way to the spatha. The scutum to a smaller oval shield. By 400 AD, the Roman soldier looked very different from his predecessor under Augustus — though the institutional continuity, on paper, remained.