AskHistorians

I'm no expert but what I remember from history class was as we declared war on Japan because of their bombing of us naval ships their allies, Germany and Italy, declared war on the u.s. in support. Roosevelt was already sending significant arms to the British. We joined in on the side of the French and British just as Germany and Italy did for Japan. US citizens did not want to join another European war after WW1 but Nazi Germany was already at our door holding rallies.

I'm sure there's more detailed information out there that isn't covered in a couple weeks of history class and retained by a high school student 20 years ago.

Germany declared war on the United States shortly after Japan did.

Germany declared war on the US shortly after the US declared war on Japan after Japan did Pearl Harbor.

As to why a focus on Europe seemed prudent, Germany and occupied/Axis-aligened Europe had substantially more industry and productive capabilities than East Asia back then while also having a similar population so for one a Nazi order in Europe could make use of extra time to prepare all manner of weapons much more effectively than the Japanese could.

Secondly the Nazis were imposing a giant threat to the British and USSR both with close proximity to their industrial heartlands. If either one is beaten or comes to the table then the situation becomes way worse for dislodging Germany. If the British become neutral then say goodbye to their vast navy, the staging ground to invade from across the Channel, and their bases and manpower from all around the world. And if the USSR loses then without the massive eastern front overshadowing everything else the Nazi’s resources can instead go to making coastal defense impassable, plus they get a massive amount of oil, slaves etc. Either of those outcomes make an amphibious invasion from the US much harder to successfully pull off. And both benefit from a boost to their morale from being told the US will help Europe first.

Japan threatening China and local European colonies was not great but comparatively less dire since those areas were way less productive than what occupied Europe was. Plus if you help Europe first then a lot of those countries with extensive holdings in Asia may feel obligated to restore their colonial territories by joining against Japan. The same doesn’t necessarily hold true for if you went the other way around first.

All that said, there were points in the early stages of America’s entry into WW2 where the war in the Pacific was taking up more bandwidth depending on what was going on and what actions they were able to perform where.

FDR wanted to get involved, but he knew that the American public didn't want to go to war. There was a lot happening before the war like the Lend-Lease Act where the USA was funding Allied war efforts while officially remaining neutral. Having a burning naval base in Hawaii and an official declaration of war from Germany and Italy gave FDR a wide berth public opinion wise to prosecute the war the way he wanted to.

Also, while there was a "Europe first" policy, the USA was active in the Pacific Front soon after the attack as the USA had to respond to further Japanese invasions including the Philippines, which was still American controlled. The USA was able to turn the tide on Japan and push Japan back to the home islands while still dedicating the majority of its war effort to Germany. The only time when the USA needed to significantly pull resources to the Pacific Front from Europe was for the planned invasion of Japan.

England brokered a deal for America's involvement.