The US battle line lacked battlecruisers and was slower than those of the other major powers. Fast cruisers could deploy ahead of it, scout, shadow an enemy, and keep pace with carrier forces that assumed some battlecruiser functions. The Portland class also had space to serve as a fleet flagship. Speed, range, aircraft, and command accommodation followed directly from this operational problem. These ships were meant to find an opponent and guide the slower battle line into action, with armour taking a secondary place in the design.
Chapter · 5:39-7:24
Fast scouting cruisers for a slow battle line
The design put speed, long-range scouting, flagship work, and carrier support ahead of the armour needed for cruiser gunfights.