Militarism is the belief that a strong military force should be adopted, maintained and used aggressively to defend or promote national interest.
The significance of militarism is that right up to the 20th century it has been a major determining factor of which civilization is more powerful than others.
Civilizations have risen and faded based on war, people have ruled or been ruled based on the strength of their military. From the 1st century up to the 20th century, militarism determined which kingdoms and empires survived.
It is only in the 21st century that things have changed such that it is economic power first rather than military power that determines the greatness of a nation.