The military history of South America can be divided into two major periods, pre- and post-Columbian, marking the entrance of European forces to the region. The sudden introduction of steel, gunpowder weapons and horses into the Americas would revolutionise warfare. Within the post-Columbian period, the events of the early 19th century, when almost all of South America was marked by wars of independence, also forms a natural historical juncture. Throughout its history, South America has had distinct military features: it has been geographically separated from many major military powers by large oceans; its unique terrain has imposed major logistical challenges, and privileged naval lines of communications. Finally, since independence few other continents have seen internal politics as closely intertwined with military affairs.