The Germans were surprised but generally pleased. Japan had not informed the Germans that it intended to attack Pearl Harbor; they learned after the fact.

Germany had seemed to wish to keep the United States out of the war through the early part of 1941, but as the year dragged on, that strategy seemed to change. Japan’s ambassador in Berlin, Baron Hiroshi Oshima, sent the following message to Tokyo on November 29, after a meeting with German foreign minister Joachim von Ribbentrop:

Ribbentrop opened our meeting by again inquiring whether I had received any reports regarding the Japanese-U. S. negotiations. I replied that I had received no official word.

Ribbentrop: "It is essential that Japan effect the New Order in East Asia without losing this opportunity. There never has been and probably never will be a time when closer cooperation under the Tripartite Pact is so important. If Japan hesitates at this time, and Germany goes ahead and establishes her European New Order, all the military might of Britain and the United States will be concentrated against Japan.

"As Fuehrer Hitler said today, there are fundamental differences in the very right to exist between Germany and Japan, and the United States. We have received advice to the effect that there is practically no hope of the Japanese-U. S. negotiations being concluded successfully, because of the fact that the United States is putting up a stiff front.

"If this is indeed the fact of the case, and if Japan reaches a decision to fight Britain and the United States, I am confident that that will not only be to the interest of Germany and Japan jointly, but would bring about favorable results for Japan herself."

I: "I can make no definite statement as I am not aware of any concrete intentions of Japan. Is Your Excellency indicating that a state of actual war is to be established between Germany and the United States?"

Ribbentrop: "Roosevelt's a fanatic, so it is impossible to tell what he would do."

Later in the same message:

"Should Japan become engaged in a war against the United States Germany, of course, would join the war immediately. There is absolutely no possibility of Germany's entering into a separate peace with the United States under such circumstances. The Fuehrer is determined on that point."

You can read the full message on this page, which includes loads of intercepted Japanese diplomatic cables. The Americans and British could decipher Japan’s diplomatic messages at the time, so any message to the Japanese embassy in Berlin with information about the attack on Pearl Harbor would have unwittingly informed the Allies as well. Japan didn’t need to tell its diplomats about the attack, so it didn’t do so.

This discussion is important, though, because you can sometimes find accounts that seem to claim that a crazed Hitler declared war on the United States after Pearl Harbor without thinking it through. Clearly, this was a possibility that was well-considered at the time.

I don’t have Ian Kershaw’s book (perhaps someone else can shed more light), but this account indicates that Hitler was “jubilant” when he heard Japan had started the war against the United States. You can read Hitler’s speech on December 11 in which he declared war, but it’s long and rambling.

After the war, Hermann Göring was interviewed by American intelligence, and said this:

Hechler: Why did Germany declare war on the United States?

Göring: I was astonished when Germany declared war on the United States. We should rather have accepted a certain amount of unpleasant incidents. It was clear to us that if Roosevelt were reelected, the U.S. would inevitably make war against us. This conviction was strongly held, especially with Hitler. After Pearl Harbor, although we were not bound under our treaty with Japan to come to its aid since Japan had been the aggressor, Hitler said we were in effect at war already, with ships having been sunk or fired upon, and must soothe the Japanese. For this reason, a step was taken which we always regretted. It was unnecessary for us to accept responsibility for striking the first blow. For the same reason, we had been the butt of propaganda in 1914, when we started to fight, although we knew that within 48 hours Russia would have attacked us. I believe Hitler was convinced that as a result of the Japanese attack, the main brunt of the United States force would be brought to bear on the Far East and would not constitute such a danger for Germany. Although he never expressed it in words, it was perhaps inexpressibly bitter to him that the main force of the United States was in fact turned against Europe.

Of course, this might be a justification after the fact, and after Germany had lost the war.

It is true that the Germans and Americans were in the midst of an undeclared war at sea in late 1941 even before Pearl Harbor. The US Navy was conducting “neutrality patrols” in the Atlantic at the time with warships to escort convoys bringing supplies to the UK. The ships were initially told to not engage German submarines, but to report them to the British. German orders at the time were for U-boats to avoid attacking American warships and US-flagged merchant ships, though this was sometimes easier said than done.

For example, the Greer Incident occurred in September of 1941. The accounts are a bit muddled, but according to Michael Gannon’s “Operation Drumbeat,” an American destroyer named USS Greer (she was a Wickes-class destroyer, same as many of the ships delivered to the Royal Navy in the “destroyers for bases” deal) was operating with a British antisubmarine aircraft. The British aircraft dropped depth charges on U-652, which believed the American ship had attacked it and fired a torpedo at it. The Greer responded in kind. Both the destroyer and the U-boat escaped without damage, but the short combat was seen as an escalation in the Atlantic war. Afterward, American warships were ordered to engage German submarines if they encountered them. In October of 1941, two American destroyers were torpedoed by German U-boats in the Atlantic in two separate engagements; the USS Reuben James was sunk.

Even if Göring was telling the truth in his postwar interview, the Kriegsmarine certainly had prepared for the possibility of American entry into the war. The German navy was waiting for the chance to engage American merchant ships and warships, and U-boat commanders were frustrated by the rules of engagement. There were plans ready to send German U-boats to the American east coast, and the first submarines sortied from France on December 18.

This isn’t pertinent to the German reaction to Pearl Harbor, but it’s worth noting here that a German living on Oahu named Otto Kuehn had a small role in helping the Japanese espionage efforts leading to Pearl Harbor. The attack would have been fine without him, but I wanted to include this in case someone asks about whether it indicates the Germans had a role in the attack. The German government did not participate in the attack on Pearl Harbor or have any knowledge of it.

My amateur research has mainly focused on the Pearl Harbor attack itself and some of the naval combat of the war, but I hope that helps! Hopefully some other posters can also provide more information about some of the inner workings of the German government after the attack.