Question

at the arcadia conference in january, 1942, what argument persuaded president roosevelt and prime minister churchill to pursue a \europe - first\ strategy?
germany was threatening great britain and the soviet union.
military leaders felt it was necessary to focus on europe first.
the us was preparing to fight japan, the strongest axis nation.
the allies had to stop germany from sending reinforcements to japan.

Explanation:

At the Arcadia Conference, the argument that Germany was threatening Great Britain and the Soviet Union was a key factor. Great Britain was already engaged in a desperate struggle against Germany. The Soviet Union, after the German invasion (Operation Barbarossa in 1941), was also in a critical situation. If Germany were to defeat either (especially Britain, which was a key Allied power for maintaining naval and air bases for operations against Germany), the war would be much more difficult to win. Focusing on Europe first made sense as defeating Germany would remove a major Axis power. Military leaders' views (while relevant) were likely influenced by the strategic threat Germany posed. The idea that the US was preparing to fight Japan (the strongest Axis nation) is incorrect as Germany, with its larger military in Europe (more troops, advanced weaponry like U - boats, Luftwaffe), was a more immediate and larger threat. Also, there was no real indication that Germany was sending significant reinforcements to Japan.

Answer:

Germany was threatening Great Britain and the Soviet Union.