This is how the Roman economy funded military expansion
In the early days of Rome, the city collected its own taxes. They would assess an individual's wealth, impose a 1% tax, and then place them into a property class. The higher your wealth class, the more you paid in taxes, which were then used to buy equipment for the military. In the event of an emergency, taxes were raised to 3%.
Later, the Empire relied more on trade and conquest for taxes than passing the expenses onto the individual. As new provinces were added to the Empire, new tax opportunities came with them. By 167 B.C., it was no longer necessary to impose a Wealth Tax on Italian mainland citizens — they still had to pay all the other taxes, though. The Romans engineered a civilization that was able to collect and distribute taxes without a central bank.
As is the case with every great force, the Roman legions needed supplies and payment. Here's how the Empire was able to raise and move the funds needed to continue conquering.
A Roman sesterce, an ancient Roman coin, had the buying power of about $1.50 USD when adjusted for inflation. Keep this rough approximation in mind when evaluating the following breakdown of Roman taxation.
The government's spending per year was an estimated 20 billion HS (sesterces). This large sum, mostly, went to supporting the standing army of 300,000 men, which accounted for 30 legions across the Empire.
The Romans exported millions sesterces, precious metals, and goods to Arabia, India, and China. Hundreds of merchant ships sailed across international waters to provide a return on investment worthy of Imperial Rome. The government imposed an import tax on these goods, netting enough return on investment to keep the troops on the war path. Towards the end of the empire, taxes on imports could be as high as 1/8th of the value of the cargo being transported.
International trade routes generated large, taxable income but any drastic change in foreign powers made these trade alliances vulnerable, and in turn, the Empire itself vulnerable. For example, when the Han dynasty fell in China, it caused irrevocable damage to trade routes to East Asia. The loss of trade partners due to foreign instability caused further strain on the ability to pay Rome's armies.
My taxes paid for that horn!
(Matthew Jose Fisher)
Conquering provinces to increase taxable territories
Conquering provinces was so lucrative that a general would go bankrupt raising an army in hopes that his invasion would pay his debts with interest, which it usually did.
Soldiers were divided into squad-like elements, called contubernium, that consisted of 8 legionaries. Each contubernium had a baggage train of one or two mules to carry heavy equipment and two slaves. A legion would have 4,000 contuberniums that would consume 8,000lbs of food and 12,000 gallons of water per day.
Troops would routinely forage for fodder, firewood, and water, but would be vulnerable to ambushes when doing so. To reduce the risks of foraging and ease the burden of paying for supplies, generals would order troops to pillage towns or population centers while awaiting resupply.
Do you accept payment in war trophies?
Supply trains traveled to pay and feed the troops
Strategic bases, usually with access to the sea, is where the payment (from taxes) and supplies flowed in from the capital and were injected into the Roman war machine.
Supply trains would go through a strategic base, through operational bases, and finally, arrive at tactical bases. Operational bases were re-purposed tactical bases that were left behind with a garrison. The new purpose of these bases was to provide security for future supply trains after the army pushed forward on a campaign. The tactical base is the end of the line, where salaries and supplies met soldiers.
Veterans of O.I.F. and O.E.F. will recognize the similarities to our logistics regarding Forward Operating Bases, Patrol Bases, and everything in between.
4 sesterces = 1 denarius