Posting in pre-1900 cause it probably needs a 19th c. POD
What's the best way to include the US in the 1871-1914 European political rivalries and alliance systems early on, so that when (a version of) World War I breaks out, US participation is full and immediate? Feel free to tweak our succession of presidents and our foreign policy priorities (because this will likely be necessary). Also, the WWI alliance blocs don't need to be the same as OTL.
The only way the US is going to get itself deep enough in "forgein entaglements" to the point of actually being in the alliance structure is if somebody is intruding into the Western Hemisphere enough that they feel their interests are actually at risk (otherwise any such alliance would be too one-sided). Post 1871, that threat can only come from Great Britain, but then we're running into the problem of "Splended Isolation" having limited British desires to get involved in Continental alliances... a fact which is likely only going to be agrivate by a rivalry with the US.
One possible solution to this issue may be a compromise Bismarkian-expansionist policy triumphing in Germany that results in them staying out of the colonial game and focusing on regional political control instead; adopting the idea that they're just big enough to still be tolerated by Europe and focusing on finding a way to limit Russian influence expansion so they can safely build up their sphere of influence in Eastern Europe and the Balkans without bumping up against any other of The Big Four's existing vital interests. A vital factor in this may be trying to back agrivate Anglo-Russian tensions and taking part in the British attempts to form a cordain to contain Russia as part of "The Great Game": maybe joining in their efforts to influence and modernize Japan, convincing and backing Britain in an intervention/mediation on the Ottomans' behalf in 77', ect., resulting in a great "Ring around Russia" alliance including AH that results in France and Russia aligning earlier. Have Britain, Germany,and Japan adopt a "Closed Door" policy on China, Britain and Germany both pushing into Latin America economically as a market for industrial output (including gunboat diplomacy and political meddling) frustrate US ambitions in Asia by facilitating a German aquisition of the Phillipenes, while France and Russia push to open things up and adopt a more liberal economic approach, and you might see a combination of aligning interests strong enough to get the US to back the 3rd Republic if the Anglo-Germans combine their naval and industrial policy enough to become a true threat to American economic hemispherical hegemony
So,you have these alliances
"League of the Five Emperors": Great Britain, Germany, Austria-Hungary, Ottomans (With more intact Balkans) and Japan
The Entente Liberal: France, USA, Russia, probably Italy, perhaps a China depending on how things pan out, and minor allies of each
