In general, these countries used two types of tactics.
First, they used economic tactics. The industrialized countries could use their economic power to come in and take effective control of the smaller countries that they wanted to control. A good example of this is how American sugar planters took economic control of Hawaii before Hawaii actually became part of the United States.
Second, they used their military power. A good example of this is how England used military power to defeat China in the Opium Wars. This allowed England to get control of Hong Kong and to gain other concessions from the Chinese.