Why Indo-European languages spread so wide?

From India to Iran, from Russia to Britain, and the whole new world, Indo-European languages spread far and wide. How so?

Indo-European languages originated from an area between Black Sea and Caspian Sea, north of Caucasus Mountain. Between two large water bodies, the air is more moist, the temperature is more temperate and the land is more fertile than surrounding areas. Outsiders will repeatedly attempt to conquer and take over this land. Earlier inhabitants, once defeated, have to look for new habitats. These war hardened people often conquer other people easily. Waves after waves of people originated from this area take over larger and larger areas. This is how Indo-European languages spread.

A fertile area surrounded by less fertile areas. People from less fertile areas attempt to take over more fertile areas. This is often why wars are fought and people move. But why this particular area between Black Sea and Caspian Sea is so fertile to generate languages and people. I guess this is because the area backs into the high Caucasus mountain. There is no easy escape for the inhabitants. The fights must be extremely fierce. Those who trained in this area must be great fighters. That is why the descendants from this area conquered so much land on the earth.

White people are called Caucasians. They are originated from Caucasus mountain. The spread of Indo-European languages is the spread of Caucasians.