The Staffordshire Regiment was formed in the King's Head in Lichfield in 1705.
When they heard the landlord was retiring and it may be turned into a bed and breakfast, veterans and well-wishers raised almost Â£65,000 to take over a 10-year lease.
A procession marked the opening of the pub, in Bird Street, on Saturday.
Veteran Geoff Pearce said: "Someone that lives locally saw the building advertised for sale - the outgoing landlord was retiring and, rumour had it, it was going to be turned into a bed and breakfast.
"I had a phonecall off one of my colleagues asking 'how do think about us buying the lease? I thought he'd lost the plot.
"The more I spoke to him the more he convinced me it could be a goer.
"It took two weeks for over 350 of the ex-members and the families to pledge Â£65,000. That's how much it meant to the veterans and the residents."
Steve Knight, a former corporal, said it was an "honour" to be the landlord.
Built in 1408, the pub is the oldest in the city. Colonel Luke Lillingston raised a regiment of volunteers in the pub on 25 March 1705.
In 1782 it was given the title of the 1st Staffordshire Regiment.
It has been used a meeting place for soldiers and veterans ever since.
Army veterans have taken over the pub their regiment was formed in 312 years ago after it was at risk of closure.