USS Lexington (CV-2), nicknamed "Lady Lex",
[1] was the
name ship of
her class of two
aircraft carriers built for the
United States Navy during the 1920s. Originally designed as a
battlecruiser, she was converted into one of the Navy's first aircraft carriers during construction to comply with the terms of the
Washington Naval Treaty of 1922, which essentially terminated all new
battleship and battlecruiser construction.
To be more accurate, the ships were not "converted", as much as repurposed during construction.
The treaty did not actually terminate new construction, as it put caps on how many of each class of ship a nation could have. It placed a moratorium on new construction, but only for 10 years. But it did allow for ships already under construction to be repurposed to other ships, like aircraft carriers. Then once the building moratorium was released it placed limits on what nations could build, as by that time most would want to retire their older ships and replace them with newer ones.
And there were more ships being tested than just "Aircraft Carriers", like the CV class. Even before the war began, nations were playing with "Seaplane Tenders". France, the UK, the US, Japan, Italy, all were playing with this concept. The US was even building new ships of that class during WWII like the Currituck class.
Plus the earliest carriers and those ships converted during construction would not count against tonnage limits so long as they were made into carriers. This simply made that class the most effective use when making such changes.
And interestingly enough, this also impacted things in WWII. As nations like the US and Japan were only recently starting to really get serious about carrier design and finally built purpose built carriers from the keel up. However, the US was still hard at work designing lighter carriers as the WNT stated that carriers of 10,000 tons or less did not count against the ship limits that a nation had. The US spent much of the 1930's designing and refining designs of such ships, which came in handy when war broke out in Europe as it could quickly build them and put them to sea.
Especially as by the end of the war the US was able to launch an escort carrier in a year (conversions in under 6 months).
And ultimately, while many concentrate on the fleet carriers, I would argue that it was the escort carriers that made the largest difference. There were so many that most convoys could actually have carrier support. And especially in the Pacific allowed the US to dominate in the area.
At the Battle of Leyte Gulf, Japan had 1 fleet carrier, 3 light carriers, and the two Ise class hybrid carrier-battleships. The US on the other hand had 8 fleet carriers, 8 light carriers, and a staggering 18 escort carriers.