Japan starts the series of conflicts in 1931 with the invasion of Manchuria (Manchurian Incident). When Japan's South Manchuria Railway was blown up, the Japanese military used the incident as the focal point of annexing Manchuria, claiming the destruction was caused by Chinese dissidents. Some point to this as the beginning of the Second Sino-Japanese War, even though Japan doesn't wage a full scale invasion until 1937 when they capture Beijing, Shanghai, & Nanjing.

Italy is next on the list with their invasion of Ethiopia, in which Mussolini used a skirmish at  Ualval as the pretext for the war. Italy lost their Ethiopia colony in Africa at 1896 Battle of Adua, and this was their chance at glory and revenge. The Ethiopian city of Adowa was ordered to be bombed and gassed with the poisonous mustard gas. Later Albania was invaded by the Italians in 1939

Meanwhile in 1936 Spain was embroiled in a civil war between the Nationals and the Popular Front. Gen. Francisco Franco conducts successful military uprisings in Morocco and Seville. The Popular Front government is supported by the British, the French stop support and Gen. Francisco Franco receives military aid from Adolph Hitler. 

In 1933 Hitler begins his reign by orchestrating the burning of the Reichstag, Germany’s parliament building, blaming the communists. Less than a month later he becomes the dictator of Germany. One night in 1934 the "Night of the Long Knives" transpired where leaders of the SA and political opponents were rounded up by the SS, 77 men were executed on charges of treason. Hitler quickly builds an army using conscription which was in violation of the conditions of the Treaty of Versailles. In 1937 Austria is annexed followed in 1939 by the invasion of Czechoslovakia and Poland, in which the next day Britain & France declare war on Germany, starting WWII.

Joseph Stalin and the USSR was also active in the 1930s, they were responsible for the 1932 genocide in the Ukraine. Starting in 1936 the Communist Secret Police arrested all the leading political figures in the Soviet Union who were critical of Stalin, by 1937 Stalin began purging the Red Army's leaders believing they were about to attempt a military coup. 30,000 members of the armed forces were executed. 
In 1939 the Red Army invaded Eastern Poland 16 days after the Germans invaded western Poland and the attempted annexation of Finland in 1939, known as the "Winter War", in which the Russians expected an easy victory but were met with stiff resistance from the Finns.

The Versailles treaty, which ended the Great War, can be directly attributed to the skirmishes and battles in Northern Europe, leading up to and including WWII. Germany, Austria (via Austrian Adolph Hitler) and Russia were fighting to get the land back that they lost in the previous war. The Treaty of Versailles took from Germany 1/8 of all its land, all of its colonies and all overseas financial assets. Russia was forced to give back the lands it took from Estonia, Latvia, Lithuania, Finland, and Poland.

The League of Nations, which was formed in 1919 under the Versailles Peace Conference, had proven to be an organization without any teeth. They sat by and watched Italy invade Ethiopia as the Ethiopians were pleading for assistance. Nothing was done when Hitler began conscripting an army and did little more than complain when Hitler reoccupied the Rhineland.

Other conflicts in the decade include Turkish troops moving into Persia to fight Kurdish insurgents, the Chaco war between Bolivia and Paraguay, the Nationalist Uprising in Egypt, and Military coups in Chile & Greece.

Territory lost by Germany, Russia, Austria-Hungary & Bulgaria after WWI