7 April 1906 = Mount Vesuvius does not erupt as it did in the OTL. Changes deep underground move the magma and create new pressures in the area.
9 November 1943 = A massive eruption of Campi Flegrei near Naples, Italy takes place. Naples and other nearby settlements are erased. The resulting tsunami obliterates all of the warships in the Mediterranean. Earthquakes rattle the region adding to the chaos.
The resulting ash clouds and dimming of the amount of the Sun's light reaching the Earth's surface leads to major disruptions in human civilization. The combat on the Eastern Front dies down as dropping temperatures make the winter harsher than any in memory. Air operations are cancelled by many powers in Europe due to the ash. Bombs stop dropping on cities for a short time. Rome is destroyed, the Fascist government destroyed. Many other Italian cities are devastated and in chaos. Norther settlements come under direct Nazi control, southern ones are on their own.
1943 = Heavy snows and a late spring in England causes major disruptions in war operations. Fields across the Balkans and Ukraine as well as other regions are unable to produce crops due to late spring and heavy layers of ash. Russian offensives against Germany slow as the summer progresses. The need for increased food production and the terrible weather conditions force many armies to withdraw.
The Second World War takes a decade off as the various nations fight famine and disease. There is sporadic fighting but no major battles except for the Americans and Japanese in the Pacific.
1953 to 1957 = The Soviet Union is on the offensive again. While consolidating their hold on Europe and finishing the bulk of their genocidal plans the Nazis managed to stockpile considerable number of rockets and a few squadrons of jet powered aircraft. These weapons allow them to stall the Red Army and force Kruschev to the bargaining table in 1957. Britain is still in a terrible state, much of India lost in rebellion and extremely harsh winters have kept it economically depressed. They are in no position to help their Russian allies.
During this time period China has broken into three regions; Communist, Nationalist, and Japanese dominated.
1960s = The United States, along with Canada, Mexico and a number of other countries, forms the North American Treaty Organization to defend themselves against further aggression by the Third Reich. As politics shift in Europe, Germany forms the European Reich, giving more autonomy to some regions of Europe but retaining control of the European Council that makes up the Reich.
Global sea-levels have reached their low-point in the 1960s. They begin to recover as the volcanic winter beings to end.
1972 to 1975 = The Chinese War takes place as Red China and the Soviet Union attack Japanese Manchuria. The United States aids the Nationalist Chinese against the Reds and Japanese in a three-way war. Communist China collapses as Bejing is destroyed by a Germany nuclear weapon - the first time such a weapon is used in war.
1980 = The United States elects a more isolationist President after the high price paid in the Chinese War. In addition to war fatigue a major downturn in the economy creates a new depression.
In the 1990s NATO increases its military spending and introduces new amazing technologies. Britain is recovering from two decades of hellish winters and is becoming an economic bright-spot in Europe thinks to American support and intervention.
1996 = Der Führer dissolves the Euro-Reich and opens Europe to free multi-party elections. This action surprises some, but the decades of economic disfunction and political unrest finally forced some action on Germany's part. It was luckily a peaceful event, der Führer was a child during the volcanic winter and war years and did not want to be responsible for a nuclear war and winter.
1996 to 2006 = Numerous small-scale wars in the Balkans and Eastern Europe.
2000 = Alaska is admitted into the United States as the 50th State.
2014 = The world is in relative peace. A new Führer in Germany takes power on March 14. He appears ready to begin reversing some of the reforms of recent years and yearns for the re-acquisition of lost territories.
Some information on Campi Flegrei and its supervolcano: