Effects of World War I:
Russia - Becomes the world's first Communist state, leading to a civil war which kills millions. Later on, becomes the main opponent to Nazi Germany, and then the US during the Cold War.
Germany - Strong sense of Nationalism begins, after what is seen as an unfair settlement (especially the crushing reparation payments). Becomes increasingly right-wing, as a reaction to the Socialists who signed the Treaty of Versailles. This all would culminate to start World War II.
France - Bitter rivalry with Germany continues; builds the Maginot Line which basically destroys from 'elan' (offensive spirirt). An entire generation of men killed; desire to avoid a war at any cost results in the harsh Treaty of Versailles which causes the opposite. Maintains a strong army following World War I.
Britain - Like France, Britain loses almost an entire generation of men. Resolved to impose harsh conditions on Germany, but realised that this would only result in war later on. Negative public reactions to war mean massive cuts in military expenditure, meaning that Britain is unprepared for World War II.
USA - Massive economic boom due to military production results in a 'Golden Age' with massive investment into companies. Eventually the bubble bursts, causing a massive worldwide economic depression; this leads to Hitler's rise to power.
Italy - Gains little for it's contribution to the war, and is left with a strong desire for colonies in Africa. Strong anti-Communist feelings make Mussolini's rise to power relatively easy. His future alliance with Hitler provides the basis of the Axis.
Japan - Joined the Allies, but recieves little in the way of German Pacific colonies. Expansionist government soon comes to power, leading it to align itself with the Axis and open up the Pacific front of World War II.
British Commonwealth - Australia and New Zealand finally have their own 'national spirit' following the landings at Gallipoli; the 'Colonial' troops prove to be far better fighters than British conscripts. This increasing independence from Britain is of little consequence until World War II, when America becomes the new world leader.
World War I ended suddenly, but the 25 year peace that followed was basically an interlude till World War II in which technology advanced further, allowing for more bloodshed.
