When Pietro and his sister Wanda Maximoff collapsed at the door of the Xavier School for Gifted Youngsters, they were almost dead. They had been travelling across Europe in search of others like themselves. When Pietro was asked how they crossed the Atlantic Ocean, he replied “We ran.”
After Pietro recovered, Professor X began training him to join the X-Men. His mutant ability to run at extreme speeds could prove very useful on the field and a special pair of silver boots were designed. They wouldn’t wear out and they could halt him if he ever went too fast. Occasionally, he would continue running and not realise he’d traveled hundreds of miles before stopping.
However, the analysis of Pietro and Wanda's mutations also revealed something else - they were the children of Magneto! Fathered in Europe many years ago, their mother Magda had sent them to find Magneto, assuming him to still be at the X-Mansion. When they discovered their father had defected and formed a terrorist brotherhood of mutants, their loyalties were torn.
Pietro decided to stay with the X-Men but Wanda defected to the Brotherhood. During a battle between the two, it was revealed Wanda was secretly trying to destroy the Brotherhood from within.
Wishing no longer to be involved in the personal battles of their family friends, Pietro (along with his sister) joined the Avengers, rather than either side of the mutant conflict.
SEE ALSO - Magneto and Scarlet Witch