In most countries, they simply conscripted them. There was no "convincing". Australia is one of the exceptions, although certainly not because the government didn't want conscription.
But who conscripted them and dragged them off to war? Was it the politicians who decided to go to war? The "convincing" isn't necessary once it's reached critical mass
There were exemptions but it still seems a cruel form of gambling in a way.
There was a military draft in WW I:
>...By the end of World War I, some two million men volunteered for various branches of the armed services, and some 2.8 million had been drafted.