Ironman holds the american flag because...
Tony Stark's real-life model Howard Hughes was an archetype of American individualism as well as a significant defense contractor who helped develop new weapons technologies.
Tony Stark/Iron Man's reliance on technology and intelligence, rather than the chance transformations of many other superheroes, reinforced the American faith in technological solutions to the military, political and ideological problems of the Cold War. Stark is an idealized portrait of the American inventor. By the 1960s, military weapons development was firmly in the realm of Big Science, with little role for the lone inventor. Issues of autonomy and government intervention in research and questions of loyalty — which real-life American physicists and engineers were also facing, if less dramatically — are prominent themes in early Iron Man storylines.
Thank you! That says it all!