It was upon finely-honed precipices that the biological survival of Homo sapiens was balanced upon, and the proud flagging bastions of the United Nations Space Command Defense Force were beset by foreign warships, the myriad cities of Earth herself metastasized by legions of inhuman foes. The atmosphere, formerly the ozone shield that defended the frail humans of Earth from ultraviolet radiation and the mercy that blessed the Earth with nurturing rain and gentle sunlight, was now an infected carrier, dusting literal tons of radioactive isotopes upon the fractured planets, remnants of destroyed UNSC warships in orbits, its winds carrier pigeons that spread iodine-131, cesium-137, and strontium-90 against the groundside defenders. The name of the engagement was the Second Battle of Earth.