Used in this context, interest means advantage or benefit. The concept of national interest can be likened to a person s self-interest.
Most people act out of self-interest.
They base their actions on what will be to their benefit or advantage. However, not all people define self-interest the same way. For example, some individuals value financial success more than anything else, so they base their actions on increasing personal wealth. Others value religious ideals and act according to their faith. Still others value power and seek ways to increase their authority over others.