Nicolo Machiavelli advises the Prince, that the only way to not get invaded and in consequence loose his position, and in case of war be assisted by many alliance armies,
is creating a wealthy and developed land, where all the neighbouring kingdoms, ask you for advice to progress as well, giving you their military assistance. Machiavelli said as well that there is not enough army to protect a poor and miserable place where the citizens are unhappy, without the essentials. “There my Prince“ he said you will be raising your lands, power, and head!
A developed country doesn’t need to loose money and human resources with its own army, they will have all the soldiers and weapons of their allies!