Employ mercenary groups to fortify your national armed forces with battle-hardened professionals. You can also carry out "unofficial" military operations without being held directly responsible by international bodies. But be careful: if you are discovered to be the one giving the orders, it could be disastrous for you.
Violent and feared by all, mercenaries are highly effective in combat. They are equipped with air transport and armored vehicles.
A dozen mercenary groups from various countries can sell you their services in an official capacity or in secret: after finding and contacting one of these groups you can employ them for a period of a few weeks to a few months.
You will control the leader of the mercenary group and can deploy their forces on the ground as you see fit.
The higher the mercenary group's reputation in combat, the more effective in combat they will be but their price will also be higher.
Each group has a secret HQ in their own country. Equipped with military cargo planes, the group can be quickly deployed into various theaters of operations where they can establish temporary camouflaged camps from which they will launch their operations.
But stay vigilant because these groups, motivated solely by money, will sell their services to the highest bidder and your enemies can use them against you!