Imperialism, was base on the principal of nationalism, exploration, expanding, also militarism, thus expanding cause the country that are already developed countries ( ie. Britain, France, U.S), to expand there country to gain power and wealthy by controlling smaller territories.
So let take it from a perspective, you would not want someone to just come in to your place and take your stuff, and you can't do anything against it.
That is in addition to militarism, the developed countries, are more advance that the countries that they will be taking over, since due to a higher weapons advance, the victims of those advance countries can't do anything, except try and resist, but ultimately they will fail for those country that tried to resist.
