After the First Drake War, many feral monsters and city sized kaiju were corralled to large unpopulated areas of Asia and Russia using pheromone towers.
In 2065 The New United Nations and North Chinese Republic joined forces to build massive walled off areas in order to protect the remaining populated areas.
Citizens willing to join the military and cross the wall can exchange time served for repayment from the new government. It can be anything from time off prison sentences to forgiveness of medical debt. The more monsters they slay, the sooner they can return home. Not many make it back, since a single monster can take on a whole squadron of mercenaries.
However, there is something of an urban legend among the new recruits. Sometimes cadets will find their entire service paid off by an anonymous donor. Nobody with long term experience hunting monsters stay any longer than they have to, so who is slaying enough monsters to pay off multiple debts overnight?
Luckily in 2075, chemical discovers lead to a decline in the amount of monsters who went feral and the need for mercenaries dwindled. The mysterious benefactor vanished, and the few records remaining say that over 300 debts were paid off, indirectling saving most of those cadets lives in the process.