Yet the Baltimore class was 13,000tons.
I'm going to ignore the Des Memes, the 8" automatics forced a significant enlargement of the ship, up to 17,250tons.
The Baltimores are built in a completely different context than the pre-Washington design studies. A non-exhaustive list of factors that drove the tonnage up that high are 1. recognition of large foreign cruisers with outsize firepower 2. wartime budgets 3. an existing mass of 10,000-ton cruisers so the ships don't need to be the whole fleet and 4. extensive experience with the Washington cruisers built telling them where their ideas had failed to pan out.
Point #2 is probably the most important one: even at that late stage there was a lot of handwringing in the USN about the Baltimores adding so much tonnage (and thus cost) for no increase in main gun firepower; without an imminent war situation during their design period and the accompanying open checkbook they might have still been built smaller or not at all.
Okay, though that suggests 5" guns on pedestals instead of 6" guns. Better rate of fire because the ammunition is lighter and can be loaded as a single unit. (And then replacing with turreted 5" guns later on.)
Correct. This was the evolution under the treaty regime basically the world over.
I suspect that the USN would still likely go to the Alaska-class to hunt whatever the Japanese built post-Takao/Mogami.
Maybe. The Alaskas were motivated in large part to fill in the tonnage gap between the battleships and 8" cruisers, but it was also a Foxbat situation where the USN thought Japan was building similar ships and they needed to match that.
Without that faulty intel the motivation is far less.
Did any other navy have an equal to the RNs Arethusa class, tiny with 3 x 2 6" guns? Or was that a result of the RNs mania with numbers arising from Britain's particular circumstances?
The two Thai cruisers that Italy took over as the Etna class are the closest equivalent - slower, but of similar tonnage and as designed identical firepower.
Aside from them Japan's late light cruiser designs had similar firepower on different lines of thought, but they wound up a fair bit bigger due to the higher speed asked of them. De Ruyter is similar but still has that extra gun. Similar designs were flogged to various minor powers but no one bit.
I think the assumption cruiser armament would converge on 8" is a fallacy. If that was the case why did everyone, literally everyone, keep building 6" cruisers, while building far fewer 8" cruisers. The point of a cruiser isn't to defeat enemy cruisers, it's to prevent enemy commerce raiding, and you can do that by shipping enough armament to threaten the other guy's chance of returning home, you don't have to sink him yourself.
They didn't. For nearly a decade after the Washington treaty only two major naval powers built any 6" cruisers whatsoever: the Germans, who were treaty-limited, and the French, who needed cruisers of all types just that badly.
Even when London kicked in, the Americans and Japanese would've been perfectly happy continuing this, which is why they were restricted in the first place!
6" lets you threaten damage to a raider while engaged in commerce protection, and blow destroyers out of the water while engaged in fleet work or functioning as a destroyer leader. You don't need anything more.
The RN considered the Hawkins a disaster because it caused the USN and IJN to focus on unaffordable 10,000t cruisers, where the RN needed the focus to be 6,000t to permit them the numbers needed worldwide.
Bluntly, this does not jive with what we know of RN thinking of the time. They wanted the 10,000-ton cruiser as much as the USN and Japan, the thinking being that director-controlled 8" guns conferred an insurmountable range advantage against ships with 6" guns in open water - and they were staring down a nightmare scenario of the Americans building
30 8" cruisers in three years.
Yet the high rate of fire those 6" guns had was valued highly enough for so many Cleveland class CLs were built even after the treaties had been made irrelevant.
In comparison the Royal Navy was much more enthusiastic about the 6" caliber and continued development of them leading to their ultimate form in the autoloading dual-purpose 6"/50 Mark N5 guns used on the Tiger class CLs.
War circumstances were involved far more than any sort of caliber preference. The RN built nothing but 6" cruisers because that was what they had building at the start of the war and once it was on there were no resources for anything fancier - none of the 8" cruiser projects got off the ground, but neither did any 6" cruiser project fancier than a warmed-over Fiji. And believe me, they wanted more 8" cruisers, especially after River Platte reminded them that their assumptions about the advantages of heavier calibers over 6" in open water were valid.
So many Clevelands, meanwhile, were built for much the same reason: as a minimum-modification of the last two Brooklyns, the Clevelands took less time to design and put in the water, a dire necessity with war imminent and the USN having wasted four years under Second London restrictions with only the four Atlantas to show for it. Notably, the Clevelands had been in combat for a year before the first Baltimores were even commissioned.
