The parallels with ww2 are far too easy to draw - the annexation, the failed appeasement attempt, the invasion.
The German invasion of Poland tends to be counted as the "start" of ww2, but the militarisation of the Rhineland, annexation of Austria, and invasion of Czechoslovakia all came first on that front. The global reach was due to European colonies abroad (especially in Africa).
The Japanese invasion of China also predated the start of war in Europe, but the Pacific war doesn't count as starting until the attacks on Hawaii, Philipeenes, Malaya/Singapore, Hong Kong etc in 1941.
Perhaps the next move is Sweden+Finland joining Nato (because clearly not being part of Nato is risky). Either way, perhaps then Russia invades the Baltics (or maybe "Belarus" does for deniability?), triggering wars all along the border. Throw in China using the opportunity to attack Taiwan while America is busy elsewhere, North Korea rolling into South Korea, and various local civil wars (probably instigated by China) in South America to distract the US further, then throw a wildcard like Syria taking a pop at Israel, and it rapidly escalates into a World War.
Historians (if any survive) may lump Crimea, Ukraine, and even places like Georgia, as the "pre-war" battles, it's arguable when local wars get enough to be deemed to be a "World War".
The Vietnam War involved at least double the number of involved countries and wasn’t considered a world war.