How was the Washington Naval Treaty enforced?

In view of Pearl Harbor...

Pearl Harbor was conducted by aircraft carriers against treaty battleships. While they were a subject of the Washington and London Naval Treaties, it was mostly to avoid exploiting a loophole by putting a flight deck on a battleship and calling it an "aircraft carrier".

In many ways the development of aircraft carriers by the US and Japan was accelerated by the treaties as money and hulls could be diverted from capital ship development to aircraft carriers and submarines. Several US and Japanese carriers were converted from battlecruiser and battleship hulls, notably the US Lexington class and the Japanese Akagi and Kaga.

I would be interested to know how the Washington Naval Treaty was enforced - if at all? - given that this was before the establishment of the UN.

At the time there was the League Of Nations which, tragically, had very little ability to enforce its pronouncements. When unanimously (except for Japan) rebuked for its invasion of Manchuria, Japan simply withdrew. The League members represented collective security, but had no provisions for actually carrying them out: they sent observers and imposed piecemeal trade sanctions. Germany, Italy, and Spain also withdrew under similar circumstances.

Studying the weakness and failure of the League will help understand the Washington and London Naval Treaties.

Since the Japanese formally terminated the treaty in the mid '30s, can it be assumed they did so because if they had not, they'd be discovered and action might ensue on part of other signatories?

Once the militant faction took over Japan and began carving up China and violating international treaties they learned nobody was going to enforce them. Unlike Germany, fearing repercussions from France and Britain while they rebuilt their military, Japan was already a regional military power. They had no heavily armed neighbors to hide from. Finally, it helped politically to publicly rebuke the perceived insult and restrictions of the naval treaties on Japan by the European powers.

By formally breaking the treaty in 1934 Japan was able to publicly build up a powerful fleet while the future Allies stayed within treaty limits until 1938 for the sake of the other (apparently) complying treaty partners. This caused a delay in the Allies fielding non-treaty capital ships, and the Allies had to fight with treaty ships against non-treaty ships early war. However, the weight of Allied ship building took its toll.

Though it should be noted that the battleships Yamato and Musashi were built in great secrecy, even after the treaty was rebuked, to prevent the US from responding in kind until it was too late. This secrecy was so great that often the designers didn't know what they were designing. This secrecy and lack of coordination between the designers, plus a hurried construction schedule to have her ready for war, lead to subtle design flaws and a long working-up period which harmed their effectiveness.

The Washington Naval Treaty, and the subsequent London Naval Treaty and Second London Naval Treaty, are rather short reads. A quick read shows it's mostly about the details with very few enforcement or inspection provisions. There are a few to avoid loopholes.

Here's one requiring treaty nations which are building ships for other nations, Britain and the US did a lot of that, to declare so.

Article XVI If the construction of any vessel of war for a non-Contracting Power is undertaken within the jurisdiction of any of the Contracting Powers, such Power shall promptly inform the other Contracting Powers of the date of the signing of the contract and the date on which the keel of the ship is laid; and shall also communicate to them the particulars relating to the ship prescribed in Chapter II, Part 3, Section I (b), (4) and (5).

Part 3, Section 1 has a bit saying they have to inform the other treaty partners if they're replacing a ship.

(b) Each of the Contracting Powers shall communicate promptly to each of the other Contracting Powers the following information...

There were provisions to modify the treaty in case of changing military or technological circumstances.

Article XXI: If during the term of the present Treaty the requirements of the national security of any Contracting Power in respect of naval defence are, in the opinion of that Power, materially affected by any change of circumstances, the Contracting Powers will, at the request of such Power, meet in conference with a view to the reconsideration of the provisions of the Treaty and its amendment by mutual agreement.

In view of possible technical and scientific developments, the United States, after consultation with the other Contracting Powers, shall arrange for a conference of all the Contracting Powers which shall convene as soon as possible after the expiration of eight years from the coming into force of the present Treaty to consider what changes, if any, in the Treaty may be necessary to meet such developments.

There were provisions in case of war.

Article XXII: Whenever any Contracting Power shall become engaged in a war which in its opinion affects the naval defence of its national security, such Power may after notice to the other Contracting Powers suspend for the period of hostilities its obligations under the present Treaty other than those under Articles XIII and XVII, provided that such Power shall notify the other Contracting Powers that the emergency is of such a character as to require such suspension.

And what happens if no agreement is reached.

The remaining Contracting Powers shall in such case consult together with a view to agreement as to what temporary modifications if any should be made in the Treaty as between themselves. Should such consultation not produce agreement, duly made in accordance with the constitutional methods of the respective Powers, any one of said Contracting Powers may, by giving notice to the other Contracting Powers, suspend for the period of hostilities its obligations under the present Treaty, other than those under Articles XIII and XVII.

On the cessation of hostilities the Contracting Powers will meet in conference to consider what modifications, if any, should be made in the provisions of the present Treaty.

And what happens if somebody wants to pull out of the treaty.

Article XXIII: The present Treaty shall remain in force until December 31st, 1936, and in case none of the Contracting Powers shall have given notice two years before that date of its intention to terminate the treaty, it shall continue in force until the expiration of two years from the date on which notice of termination shall be given by one of the Contracting Powers, whereupon the Treaty shall terminate as regards all the Contracting Powers. Such notice shall be communicated in writing to the Government of the United States, which shall immediately transmit a certified copy of the notification to the other Powers and inform them of the date on which it was received. The notice shall be deemed to have been given and shall take effect on that date. In the event of notice of termination being given by the Government of the United States, such notice shall be given to the diplomatic representatives at Washington of the other Contracting Powers, and the notice shall be deemed to have been given and shall take effect on the date of the communication made to the said diplomatic representatives.

Within one year of the date on which a notice of termination by any Power has taken effect, all the Contracting Powers shall meet in conference.

The Second London Naval Treaty addressed a few issues as international tensions increased. Primarily they included "Escalator Clauses". Capital ships were limited to 14" guns, but in case one party violated this, all parties could go up to 16" guns.

Article 4, Section 2 No capital ship shall carry a gun with a calibre exceeding 14 in. (356 mm.); provided however that if any of the Parties to the Treaty for the Limitation of Naval Armament signed at Washington on 6 February 1922, should fail to enter into an agreement to conform to this provision prior to the date of the coming into force of the present Treaty, but in any case not later than 1 April 1937, the maximum calibre of gun carried by capital ships shall be 16 in. (406 mm.).

Furthermore, if any party broke the treaty, others could also break the treaty.

Article 25, Section 1 In the event of any vessel not in conformity with the limitations and restrictions as to standard displacement and armament prescribed by Articles 4, 5 and 7 of the present Treaty being authorised, constructed or acquired by a Power not a party to the present Treaty, each High Contracting Party reserves the right to depart if, and to the extent to which, he considers such departures necessary in order to meet the requirements of his national security;

Britain and the US invoked this clause in response to rumors of Japanese super battleships.

But no enforcement or inspection clauses. It seems it was just if one violated it, they all had the option to renegotiate the treaty, or later to make "departures necessary in order to meet the requirements of his national security".

It seems to have been a sort of "mutual protection by mutual interest", both militarily and economically. At the time France and Britain were eager to avoid another expensive and ultimately fruitless naval arms race that led up to WWI. The United States was eager to build its navy up to protect two oceans, but also restrict it's main rival: Japan.

Italy saw it as an opportunity to restrict the naval power of Britain. Britain needed a globe-spanning navy, but Italy could concentrate on the Mediterranean. With Britain and Italy restricted, Italy could calculate what it might take to dominate the Mediterranean without breaking the budget.

Japan was the wildcard. To some in Japan, the Western Pacific was theirs, and the US was their great rival. They could never out-build the US, so one faction viewed the 6-to-10 ratio as a restriction on the US, not Japan. Another faction viewed 7-to-10 as the minimum ratio, taking into account the US's two-ocean obligations, and considered the treaty a stinging rebuke of rising Japanese power.

They looked for a repeat of Tsushima. The plan, Kantai Kessen, was to sucker the US fleet deep into Japanese territory to defend the Philippines, whittling it down all the way across the Pacific with aircraft and submarines, and destroy it in one climactic battle. Pearl Harbor threw that plan into disarray.

It turns out the former were correct. As Japan would painfully learn it was more a restriction on the US than Japan, and the US wasn't even building to the treaty limits prior to the war.

In a side note, the Japanese militant faction had an odd fascination with numeric ratios while simultaneously arguing that Japanese zeal would carry the day. When arguing policy and budgets, they'd point to the unfavorable US/Japanese ratios to get more ships. When arguing ambitious strategy and tactics, they'd point to zeal to make up for numbers. This cognitive dissonance in the Japanese high command, in my opinion, lead to some unrealistic strategic decisions.

Part of understanding this oversight is the nature of capital ship construction. They're big. They're really, really big. There's only so many shipyards which are capable of building them, and so many ports capable of berthing them. It was hard to keep capital ship construction secret. The original Washington Naval Treaty was more about about numbers, less about tonnage. Numbers of capital ships built and under constructions can plainly be seen.

With numbers restricted, the future Axis powers got creative. They realized it's hard to lie about numbers, but it's very easy to lie about tonnage. Article XI prohibits non-treaty warships over 10,000 tons standard displacement. Measuring a capital ship's true displacement is very difficult, and it's easy to report bogus official numbers.

This is exactly what the Germans did with the Deutschland "cruisers", the "Pocket Battleships", though in violation of the Treaty of Versailles. Officially 10,000 ton cruisers, Deutschland as 12,600 tons, and Admiral Graf Spee was 14,900 tons. The Japanese violated cruiser tonnage limits when the Myōkō-class ballooned out to 11,600 tons. The Italian Trento and Zara-class cruisers were also overweight. The British, French, and US generally stayed within cruiser treaty limits.