The Coalition of Western Republics has always had its eyes on the birthplace of mankind. While the Earth itself was an irradiated dustball filled with mutants and techno-barbarians, the Sol System was strategically placed within the Inner Sphere and regaining the old American homeland and conquering the "Graveyard of Empires" would give the Coalition massive prestige. Ever since the Coalition began to gain a galactic viewpoint, it made the capture of Earth one of its main goals.
But the final decision to invade the planet was not made until the ascension of Supreme Director Michael Applegate. Elected by the Coalition Council as a compromise, Applegate was viewed as a weak man who could easily be dominated by one of the Coalition's many political factions. It was the factional infighting that gave him his directorship that typically elected weak, old Council members and deposed those that overstepped their bounds. Applegate was determined to show the Coalition that he was a strong leader who would not let anyone get in his way. He therefore made it his administration's goal to do the impossible: conquer Earth.
Empires have been trying to conquer Earth for as long as there have been empires, but up until 2739 none of them succeeded. Even after the growth of interstellar states, Earth was too much of a hassle and not worth the price of blood and treasure needed to hold onto it. Logistically, the Sol System was hard for any of the superpowers to invade, as it was far from their core systems. The Coalition already established a foothold in the northern regions of the old Empire, but Applegate was determined and planned to use the entire might of the Coalition to subjugate the entire world.
The pretense for war was flimsy, but it did not need to be particularly strong. An Indonesian state was terrorizing the Santiaguino Empire, a successor to the American Empire. The tipping point was a surprise attack by the Indonesians against Santiaguino outposts in the core of the old Empire. Although the Coalition always regarded the Santiaguinos an illegitimate state, this gave Applegate the casus belli he needed. Applegate sought aid from Earth's neighbor, Mars. The Martians tried thirteen times to conquer the planet but were repelled every time. While they initially wanted to have the honor of taking Earth, they were persuaded by the Coalition to allow Coalition troops on their planet.
The Coalition's attack was particularly brutal. Orbital saturation bombardment was used on the planet for several months before the first Coalition boots even hit the ground. Biological and chemical agents, designed to harm even the most mutated of humans, were used liberally. During this stage, the Santiaguinos were to take the brunt of the fighting, consolidating their control over Central America and invading the Pacific and the barbarian states of Europe. The small Coalition garrisons in North America repelled attacks from the other "successor states" to the south as they were destroyed from above. The Coalition's indiscriminate bombardment killed more Santiaguinos than the enemy, which was undoubtedly intentional. The only planetside fighting the Coalition had to endure was capturing whatever cities there were left and securing the American heartland from barbarians. Whatever governments were left quickly surrendered.
The Coalition occupation was equally brutal as the invasion. Coalitionists despised mutants, having had to fight them constantly during the Endless War. The military and the DSS quickly rounded up the mutant populations and placed them in ghettos or outright exterminated them in mobile concentration camps. Among the "pure" human population, the Coalition's popularity was split. While some hailed them for uniting the planet, ending banditry, bringing the luxuries of the 28th century to Earth and enforcing the rule of law, others saw them as tyrannical oppressors that needed to be deposed. The Coalition had to put down insurgencies throughout the entire planet, carried out by separate groups who were only united by their hate for the "alien" power. Coalition troops oftentimes resorted to "draining the rivers to catch the fish," rounding up entire populations and executing them for suspected harboring of guerillas. This harmed the Coalition's popularity dramatically, increasing the numbers of guerillas and atrocities.
Applegate was ecstatic about the invasion. The relatively low casualties led him to proclaim victory throughout the Coalition, and he marked himself as the man who did what Alexander the Great and Napoleon could not. He used the opportunity to purge all of his enemies, labeling them "traitors during a time of war." However, the purges would eventually cause more destabilization to his reign, making him more paranoid and believe that there is a conspiracy to overthrow him. Determined not to let that happen, Applegate conspired with the leaders of the other two superpowers to start an even greater war.