Hang on... how does allying with Russia contain Russia?
Allies presumably do not attack each others' colonial holdings, which was the point of the Anglo-Russian alliance. It's easy with the benefit of hindsight to realize how fragile the Russian Empire was, but in the early 20th century the Russian Empire terrified most of Europe with its apparently limitless population and explosive economic growth. A potential Russian invasion of India or Britain's holdings in the Middle East or China was one of the major strategic preoccupations of British decision makers in the early 20th century, and they elected to try to make Russia an ally through diplomatic means to annul that potential threat.
The point of the Anglo-Russian Alliance was not to contain Russian aggression in other directions (i.e. against the Ottoman Empire, which was one of the Russian Empire's primary long-term geostrategic objectives, or against Germany which was mostly brought about by the Franco-Russian Alliance rather than any particular objective of Russia's), just to neutralize the potential Russian threat against Britain's holdings.
Edit: To elaborate about WW1 and what might be going the fuck on in this book's ?WW2?, on a very basic level, here's what each power going into WW1 wanted from their diplomatic arrangements.
Britain - Wanted to prevent invasions of their colonial empire, competition for more colonies, and generally preserve their position as King Shit of the World.
France - Wanted revenge against Germany (and, preferably, go back to there not being a modern united German state) and money from bankrolling other nations' armament programs.
Russia - Wanted control of the Bosphorous to link the Black Sea with the Mediterranean, either via direct control or via a pliable puppet.
Germany - Wanted membership in the Big Swinging Colonial Dick club and generally recognition as a great world power.
Austria-Hungary - Wanted southeast Europe to please stop exploding and be acquiescent pawns while A-H is preoccupied with its own internal issues.
Ottomans - Wanted everyone to stop hitting them and please loan them money and military expertise so they can rebuild.
This is what fundamentally set off WW1 with Serbia and Austria-Hungary.
Now, given what we know about this wacky alternate timeline, we know that Napoleon probably won his wars, France and Germany are allied (or even united in some capacity), and the United States does not exist except as a dominion of the British Empire.
I'd guess that yeah, WW1 probably never happened in this timeline. A victorious French Empire under Napoleon probably precludes the formation of a recognizable modern Germany, and the presence of Hitler here as part of the Franco-German forces suggests that Austria-Hungary is either gone entirely or also part of this new continental bloc. I'd assume that this war, then, is the Mega-British Empire versus a European continental bloc, either as a single unified power or an alliance of powers that incorporates what we know as France, Germany, and Austria-Hungary (almost certainly Italy as well). Which leaves Russia as potentially going either way, maybe still Tsarist or maybe the Soviet Union having arisen one way or another despite all the butterfly nonsense.
This is
not a world the Animorphs know anything about, and it kind of speaks to their prejudices as Americans that they'd assume that an invasion of continental Europe during the mid-20th century has to mean that the invaders must be the good guys.