The diffusion of religion, language, in addition to other aspects of culture historically caused by mass migrations, travel, colonization, ostracism, inter group communication, Interactions among people allowed for the creations of simplified language interconnectedness demonstrated by the developments of Lingua francas, creolization, etc.
Colonialism is defined as an attempt by one country to establish settlements and to impose its political, economical, and cultural principles in another territory. Colonialism is a violent form of cultural diffusion.
Here's an example: Christianity started in Israel but is now practiced all over the world, initially spread by the Roman Empire. Through missionaries that spread religious beliefs, Christianity is one of the universalizing religions practiced worldwide.
Christians also practiced forced diffusion using violent means to spread Christianity throughout Europe and also in the Americas. The cultural diffusion of Christianity amongst Native Americans in America was often through legal coercion, with tribes given ultamatems that they can only "own" their tribal territory if they participate in Christianity. Populations of Spanish, French, English, and Portuguese imperialists and colonialists imperialized regions of the American Sub-Continent.
The trading of goods by merchants also allowed cultural diffusion. In order to communicate with traders and consumers, common languages were developed to increase interconnectedness. Trade routes are centers of more people, which means more cultures coming together. The establishment of trading routes such as the Silk Road led to further trading and communication connections to arise.