Financing the operations of defense forces is not a cheap affair. Huge sums of money have to be spent on military hardware. More money has to be spent on personnel salaries. There are also utilities, like vehicle, tank and plane fuels to be bought, as well as other supplies like special foods for the soldiers and so on. So a question arises, as to how countries get the money to finance their defense forces.
The first way in which countries get the money to finance their defense forces is through taxes.
The second way in which countries get the money to finance their defense forces is through aid. We have situations where countries give others military aid. But this aid is seldom free. Rather, there are indirect ways of paying for it (like where the aided country has to reciprocate by protecting the benefactor country’s interests). This is akin to the Gmail.com model where, in exchange for gaining access to the www.gmail.com features, users are expected to put up with ads. The countries that receive military aid sometimes end up being very vulnerable: to an extent where the benefactor countries can go as far as determining who the leaders of the military aid recipient countries will be.