Springfield is a city in the state of Missouri.
Captain America's Comics
Gary Hendricks, a young fan of the war hero Captain America, received a copy of a comic book edited to help boost morale during World War II, that told an action-packed story about his favorite Star-Spangled Hero.
Hendricks avidly read the story where Captain America stopped a group of military traitors from stealing a newly-created bulletproof Electro Suit. Captain America, triumphant, swore that he would ever be vigilant and protective of the nation's freedom.
Hendricks finished reading the comic, and entered his home to have dinner. The mailman left Hendrick's mail near the comic, as he knew that he would be back to retrieve it.
Investigation of the Mandarin Attacks
In 2012, Springfield was the location of an unusual thermogenic occurrence, in which the thermogenic signature rose to 870 degrees Celsius.
Tony Stark started investigating the Mandarin, in order to enact revenge for the explosion that almost killed Happy Hogan. As usual, the explosive device was not found, but the heat from the blast was particularly high, more than 3000 degrees Celsius.
Stark studied the different thermogenic occurrences to study other cases based on the particular temperature of the explosion, discarding the thermogenic signature at Springfield for not being as high as that of the explosion.