I am now going to describe a legal defense created by some Imperial jurist nearly a century ago. I read it about 20 years ago; any mistakes are likely mine:
1)A very literal interpretation of Hague 1899 Declaration II:
´The Contracting Powers agree to abstain from the use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases.´
Germany deployed Chlorine gas first, but only from gas tanks; the first side to use gas shells was the Entente. Yes, this happily ignores the spirit of the treaty ... but jurists were much more literal a century ago.
2)The Allies seem to have used ´deleterious gases´ before Germany readied Chlorine. Dates and locations for the following actions were given:
French troops used the tear gas stocks of the Paris PD against German troops without diluting it. This was much more dangerous stuff than modern tear gas; it could be easily lethal at high doses.
British troops used stink bombs to dislocate German troops from their improvised position and then shoot them.
Naval Law before the Great War knew two different types of blockades: Loose ones and close ones.
A close one had no legal problems, it allowed stopping ships near their destination (like in the Napoleonic Wars), but was quite risky (mines etc.).
Britain went for a loose blockade (The loss of HMS Aboukir, Cressy and Hogue may have been a factor.), stopping ships at high sea.
But a nation was only allowed to install a loose blockade, if it was able to block all major ports of a nation with a close blockade. The RN was never able to enter the Baltic, and Germany happily traded with Scandinavia.
So a British loose Blockade would have been illegal under the pre-war rules, but British jurists started to discover new aspects of International law, while their German colleagues were much more conservative ...
Caveat: Naval Law is not my area of expertise.
Stopping and searching neutral ships creates major legal problems, to be polite.
I could also mention the illegal use of francs tireurs and the taking of civilian German hostages by French troops - admittedly not in WWI, but after WWII. It is difficult to shoot German hostages with a front line running through France.
I suppose Your bombardments by sea refer to the battlecruiser raids. These were primarily directed at defended harbours IIRC and quite legal. One hit an undefended harbour, but British pre-war planning defended it with artillery batteries in case of war. Knowing this, the Germans assumed it defended ... sorry about the mistake.
Please specify Your incidents of bombardment by land.
I suppose it was Art 10 f. of the Congo Act 1885, which neutralized parts of Africa. Britain´s legal minds limited it to the Congo itself and certain areas close by, while Germany expanded it to encompass nearly all of Africa. I just read it, and IMHO the truth lies somewhere in the middle.
Deutsch-Ostafrika was affected by said neutralization,even under the strict British definition, see Art. 1 § 3 of said act. The shelling of Daressalam by HMS Pegasus on August 8th, 1914 to destroy the radio station (a British warship shelling an undefended town - oops!) violated that treaty: The British re-defined said action as a purely naval matter ... a weak excuse.
My sources are the above-mentioned propaganda booklets I accidentally found as a young student. Comparing the British with the German ones was fun - but I digress.
I can locate them, but this would take some time. They are certainly not yet available on the net.
Do You really want to send this humble poster into the dusty cellars of Library II FB 09 FU Berlin?
The dumdum story listed lots of impartial German witnesses, of course.
It concentrated on the French 8 mm bullet design, esp. its latest version, that in their opinion only made sense as it was extremely easy to turn into a dumdum version. Germany allegedly captured lots of special devices looking like oversized pencil-sharpeners in Eastern France: Said devices had only one use according to the Germans, namely to turn 8mm bullets into dumdums en masse.
I tend to believe in a pre-dominant use of dumdums by the Entente, as Germany could not re-deploy its colonial troops to Europe, while the Entente used quite a few colonial units. A lot of Allies accustomed to using dumdums faced by Germans without such a tradition ... and now add the vicious Entente propaganda, de-humanizing the Hun.
Yes, a unique new idea, but especially the little, mean-spirited stuff like forcing the police to retire its pistols and buy new ones with shorter barrels or forcing arms control laws onto German civilians ... because Prussian cops and Bavarian hunters were going to start a war.
This trend continued in later treaties, Germany had to forbid THC-spiced cigarettes to get US credits to pay reparations. It got nearly no radio frequencies in later international conferences etc etc. etc. . Pity revenge, that did nothing but enrage Germans.
I suppose we have to agree to disagree about a history of agression.
My impression about the legalities of WWI is different, but my user name betrays my Teutonic heritage:
The Germans tried to keep the letter of a law while happily violating its spirit (Legal justifications e.g. for invading Belgium were clearly written by very competent jurists, and are in some ways quite clever, but they only give a fig-leaf.)
The Brits gave priority to winning the war and simply ignored legalities, esp. on sea.
Sorry about the lenght of my post.