(Even though chronologically, Captain America: The First Avenger and Captain Marvel precede Iron Man.)
This was evidenced by showing the S.H.I.E.L.D. logo, having Emil Blonsky reference the Super Soldier Serum to foreshadow Captain America: The First Avenger and Tony Stark visiting General Ross in the mid-credits scene.
It’s important not to forget that a month after Iron Man’s release, The Incredible Hulk came out, and although it was distributed by Universal rather than Paramount, it still served as the second MCU entry.
However, Iron Man was the first movie to kick off this trend, and combined with its critical lauding (in large part due to Robert Downey Jr’s performance), it remains one of Marvel’s most popular movies.