It often occurs that the defense forces of a given country are deployed to protect another country. Here, we are talking of a situation where the defense forces of country A venture into country B, not with the objective of attacking country B, but rather, with the objective of protecting country B. in such situations, the ordinary citizens tend to have questions as to what exactly justifies the deployment of the their defense forces to protect foreign countries.
The first justification for deploying the defense forces of a given country to protect another country is in the fact that the other country in question may be facing a humanitarian crisis.
The second justification for deploying the defense forces of a given country to protect another country is in the fact that the other country in question may be of particular strategic objective. So the stronger country offers its defense forces, to protect the weaker country, if the weaker country is of strategic importance. This it does in exchange for certain strategic favors. That is where, for instance, the weaker country that is hence protected offers the stronger country its natural resources, or where it offers the stronger country the chance to create military bases.