In 2020, the USA introduces several pieces of legislation, which are designed to lower immigration rates. They seriously offend many of the other nations in the UN, but when they attempt to pass a motion to remove the 'racially offensive' laws, the USA uses their dominance of the UN to block the motion. Several nations quit in digust, including: China, France, Germany, Japan and Russia. Notable nations that remained in the UN are: USA, UK, Brazil and South Korea.
Then a piece of bad luck struck the USA, as the EUU party (European Union Unification Party) took power in the UK, and at the EU conference in 2022, the EU became a mega-nation, as all the EU nations joined the EUN. They then peacefully annexed all the other European Nations, and the Commonwealth. The USA had just lost an ally. At a similar time, the South American countries formed a federation, excluding Brazil, to save themselves from Economic Collapse.
The war began with a dispute between the USA and EUN in North America (Canada), when a EUN citizen convicted of murder fled to the USA, where they sentenced him to death, and he was executed. This was considered illegal by the EUN, as they had abolished the Death penalty, and the crime was commited in the EUN. The EUN demanded his body, the USA refused, and they both moved troops to the border to secure their aims. The USA relented and returned the body, but as they did so, a US National Guard trooper shot and kill an EUNAFP (EUN armed forces personnel) in a bar brawl. The EUNAFP in the bar retaliated by shooting all the US troops they could find. Inoutrage, the USA declared war on the EUN, which responded by pulling their troops back to a prepared defensive position, and began to move troops from Europe to Canada. The USA moved troops to the north to prepare for an attack. The war had begun.
The USA struck first, pouring over the border into Canada. The US forces slammed into the EU defenses, and were wiped out. EU special forces are parachuted into the USA, and break the US nuclear silos, fighting there way in, and then destroying the silo door mechanism. When the USA attempted to fire the nuclear missiles, they destroy the Silos. The EU then pushed forwards, destroying the shattered remnants of the US troops in Canada. First Blood to the EU.