Here we see Columbia and the Amarillo on a joint patrol mission...
My theory is that Starfleet and UESPA were heavily outnumbered by the Romulan forces. While the United Earth forces had some vessels such as the NX class that were on a par with their Romulan counterparts, the NX class and its sister designs took a while to construct. The Romulans however had large numbers of obsolete ships like the Cabbage and started to strike at Earth's resources and trade routes. The United Earth government needed a way to construct large numbers of ships, that were smaller and quicker to construct than current designs but also contained as much if not more hardware than an NX class starship.
Their answer was the longitudinal deck arrangement and the first design was the Amarillo class. Many were fitted with readily available Warp 3 reactor. However just as Starfleet's prayers were answered the Romulans struck at Earth's main antimatter processing station. Starfleet reacted quickly to this by arming all ships with the older spatial torpedoes and refitting them with high yield nuclear warheads, thus only using the stored supplies of antimatter for fuel.
During this time the Allied governments including the Vulcans and Andorians didn't want to actively support the Humans with their military fleets, just in case the Humans lost and the Romulans sought to punish anyone who assisted Earth. They did however provide manpower and large numbers of Andorians, Tellarites and Vulcans were assigned to the fast growing United Earth forces.
As the Romulans used surprise hit and run and ambush tactics to destroy starships, stations and convoys, Starfleet wanted to provoke the Romulans into committing a large proportion if not all their fleet into a massive engagement and overwhelming them with sheer numbers. The tactics weren't all that successful with both sides winning and losing a number of small battles. In early 2260 Section 31 was approached by a Starfleet Admiral to feed the Romulan intelligence services with false information which stated United Earth forces had established a staging area in the Omicron Gruis system, known to the Romulans as Cheron. This would put a large Earth force within striking distance of Romulas. The Praetor ordered the bulk of the Romulan fleet to obliterate Omicron Gruis and catch the Humans unaware. However Starfleet and the United Earth Navy were waiting for the Romulans and outnumbered them 3 to 1. The Romulans fought to the last starship and it became the most humilating episode in Romulan history.
After that the treaty was quickly signed and the neutral zone established. The Humans had proved they could change the course of galactic events and protect their allies. It was then decided the Coalition of Planets should become a United Federation of Planets with a single exploratory and military arm modelled after Earth Starfleet since it was the largest agency, and the sharing of resources across all member worlds so the shortages faced by Earth during the war wouldn't happen in the future.
