Civil War was a battle between the super heroes that started because the Young Avengers were blamed for causing an explosion that destroyed the Empire State building.
The First Act of War
The Young Avengers were noticing that a large number of villains were in their lair. This included Annihilus, Gambit, Machine Man and a version of Ultron. Surge took on Annihilus, who then broke the steel gurders that were holding up the building. Hulkling and Nico then took on Ultron, so Nico casted a rusting spell that stopped the joints in Ultron from moving. Machine Man then took Patriot's shield and threw it at the civilians, chopping off the head of Miriam Sharpe, who was on vacation with her son.
As the battle continued, Gambit ran of to the block where the Empire State Building was. Speed followed him, as did the Human Torch. Gambit stopped at the Empire State Building and threw several charged cards at it. A explosion happened, and Speed, not realizing what just happened, created one his "booms", setting the entire block on fire. Speed left as soon as he realized what he did. The Human Torch got there, and with all these builings on fire the public thought he was to blame. The police arrived and shot him out of the sky.
The super hero community was shocked. They realized that if it was any other place in the world it wouldn't be as hard. But it was New York. The town and state that they lived in. SHIELD knew that they would have to ban the heroes or make them join the organization. On other worries, the Fantasic four was attacked by the Yancy Street Club. More super heroes were being attacked and finally the President ordered that all super heroes join SHIELD for their own safety.
Barely any sided with the idea. The ones that did were Defenders, Excaliber, Fantasic Four, Thunderbolts, the X-Men (for then), and the Young Avengers. The ones that refused to join were Alpha Flight, Avengers, A-Next, Champions, Cloak, Force Works, Freedom Force, the New X-Men, and Patriot and Hawkeye, who began dating. Also the villain teams Acolytes, Brotherhood of Evil Mutants, Masters of Evil, and Sinister Six formed a super villain army in California, where they established a base that could hold against all forms of attack, even a atomic bomb. They called this new group the Legion of Evil.