Answer: Tanks and military-grade planes
Explanation: Imperialism was about almost everywhere in Europe at the time and around that time most European countries had territories in Africa which lead to constant fighting for the land there. What this lead to was intense fights between nations by them creating proxy wars (militias/Rebel groups) to fight against each other which were being supplied by that country that wants more land for them self. With that it lead to Archduke assassination which lead countries to declare wars on one country which then would mean they would declare war on multiple countries in realty because countries would ally each other and the domino effect went down hill from that. So Archduke assassination was just really a "reason" to fight with another nation for more land (resources).
It was the Potsdam Conference that decided on how to address the issues arising from the surrender of Nazi Germany. The conference was attended by <span>The Big Three — Soviet leader Joseph Stalin, British Prime Minister Winston Churchill (replaced on July 26 by Prime Minister Clement Attlee), and U.S. President Harry Truman — in which they met in </span>Potsdam<span>, Germany, from July 17 to August 2, 1945, to negotiate terms for the end of World War II.</span>
I believe that 3 is the most concerning issue because everyone was worried that the Soviet Union or some other country would take their land.
Because there was so little known about the ocean floor at the time
