The Jovian Moons campaign began due to Jovian secessionist attacks on United Nations Colonial Advisers on the moon Io. This led to three months of fighting between United Nations and Jovian "Frieden" forces. Though this was not the first armed conflict in our Sol system, it was easily one of the bloodiest, and is generally considered to be the spark of increased friction and militarization that followed.
The Jovian Moons campaign escalated tensions, as Earth's National governments, many of which sponsored colonies within the system, began fighting proxy wars off-planet. As these proxy wars continued, tensions on Earth mounted, leading to a number of armed conflicts on Earth itself, such as the Rainforest Wars