Question: Why did the leader who broke the Molotov–Ribbentrop Pact by invading Bukovina first sign it?

Knowledge 1: Soviet Union in World War II
Germany invaded Poland on 1 September 1939. Joseph Stalin waited until 17 September before launching his own invasion of Poland. Part of southeastern (Karelia) and the Salla region of Finland were annexed by the Soviet Union after the Winter War. This was followed by Soviet annexations of Estonia, Latvia, Lithuania, and parts of Romania (Bessarabia, Northern Bukovina and the Hertza region). It was only in 1989 that the Soviet Union admitted the existence of the secret protocol of the Nazi - Soviet pact regarding the planned divisions of these territories. The invasion of Bukovina violated the Molotov - Ribbentrop Pact, as it went beyond the Soviet sphere agreed with the Axis. [SEP] Knowledge 2: Molotov–Ribbentrop Pact
On 22 August, one day after the talks broke down with France and Britain, Moscow revealed that Ribbentrop would visit Stalin the next day. This happened while the Soviets were still negotiating with the British and French missions in Moscow. With the Western nations unwilling to accede to Soviet demands, Stalin instead entered a secret Nazi–Soviet pact. On 24 August a 10-year non-aggression pact was signed with provisions that included: consultation, arbitration if either party disagreed, neutrality if either went to war against a third power, no membership of a group "which is directly or indirectly aimed at the other".
Knowledge 1: Molotov–Ribbentrop Pact
The stated clauses of the Nazi-Soviet non-aggression pact were a guarantee of non-belligerence by each party towards the other, and a written commitment that neither party would ally itself to, or aid, an enemy of the other party. In addition to stipulations of non-aggression, the treaty included a secret protocol that divided territories of Romania, Poland, Lithuania, Latvia, Estonia, and Finland into German and Soviet "spheres of influence", anticipating potential "territorial and political rearrangements" of these countries. Thereafter, Germany invaded Poland on 1 September 1939. After the Soviet–Japanese ceasefire agreement took effect on 16 September, Stalin ordered his own invasion of Poland on 17 September. Part of southeastern (Karelia) and Salla region in Finland were annexed by the Soviet Union after the Winter War. This was followed by Soviet annexations of Estonia, Latvia, Lithuania, and parts of Romania (Bessarabia, Northern Bukovina, and the Hertza region). Concern about ethnic Ukrainians and Belarusians had been proffered as justification for the Soviet invasion of Poland. Stalin's invasion of Bukovina in 1940 violated the pact, as it went beyond the Soviet sphere of influence agreed with the Axis. [SEP] Knowledge 2: Pacific War
In September 1940, Japan decided to cut China's only land line to the outside world by seizing Indochina, which was controlled at the time by Vichy France. Japanese forces broke their agreement with the Vichy administration and fighting broke out, ending in a Japanese victory. On 27 September Japan signed a military alliance with Germany and Italy, becoming one of the three Axis Powers. In practice, there was little coordination between Japan and Germany until 1944, by which time the U.S. was deciphering their secret diplomatic correspondence.
Knowledge 1: Bukovina
A region of Moldavia during the Middle Ages, the territory of what became known as Bukovina was, from 1774 to 1918, an administrative division of the Habsburg Monarchy, the Austrian Empire, and Austria-Hungary. After World War I, Romania established its control over Bukovina. In 1940, the northern half of Bukovina was annexed by the Soviet Union in violation of the Molotov–Ribbentrop Pact, and currently is part of Ukraine. [SEP] Knowledge 2: Molotov–Ribbentrop Pact
In an effort to demonstrate peaceful intentions toward Germany, on 13 April 1941, the Soviets signed a neutrality pact with Axis power Japan. While Stalin had little faith in Japan's commitment to neutrality, he felt that the pact was important for its political symbolism, to reinforce a public affection for Germany. Stalin felt that there was a growing split in German circles about whether Germany should initiate a war with the Soviet Union. Stalin did not know that Hitler had been secretly discussing an invasion of the Soviet Union since summer 1940, and that Hitler had ordered his military in late 1940 to prepare for war in the east regardless of the parties' talks of a potential Soviet entry as a fourth Axis Power.

Answer: reinforce a public affection for Germany