Of course, Japan attacked Pearl Harbor. But why wasn't there any other attacks afterwards? Didn't they have ships and planes they could have sent out here?
Were the Nazis' hands too full in Europe to attack the USA some more? And if so, why then did they even bother with America before they were ready for a full out attack?
Well, there would be three ways that the Nazis would have attacked the US: aircraft bombings, amphibious attacks, and naval bombardment.
Aircraft bombings: The US was so far away from Germany, that planes of the time couldn't reach the US mainland.
Amphibious attacks: amphibious assaults are notoriously difficult, requiring precise coordination and often sustaining massive casualties. The largest amphibious assault of all time were the Normandy landings, and they came from the comparatively close England, across the English Channel. An amphibious invasion across an entire ocean would be essentially impossible, and would damage the Nazi strategy of overwhelming force.
Naval bombardment: similarly to the last two, the common theme of distance rings here. The US is so far, and Germany as a largely land based nation, while it had a navy, wasn't a match for the US.
Its a question of means.
The IJN was the 3rd largest Navy afloat in 1941, and one that had spent 30 years examining how it would conduct a war for control of the Pacific. It had the ability, if limited, to project power across the expanse of the Pacific. And beyond Pearl Harbor was able to deploy several I-Boats off Hawaii(they even put a torpedo into the Saratoga), and off the West Coast.
The Kriegsmarine on the other hand was a rush job of a force and the result of a hodgepodge of competing ideas for a robust multi mission force and one meant to double down on commerce raiding. By December of 1941 it was a force incapable of projecting surface power effectively beyond range of land based air, and even that was limited in numbers by the loss of many ships off Norway and 1/4 of its Capital ships in the Bismarck. While its other vessels had been either chased from the seas back to friendly ports or sunk.
Thus attacks on the US fell to the U-Boats, and the long ranged Type IX boats at that as the more common Type VII's lacked the range in all but the most extreme circumstances and preparations. But the attempt was made and soon after Pearl Operation DRUMBEAT was started by sending the 5 available Type IX boats(out of about 25 operation at the time with more building) to North America, where the lack of sufficient ASW assets and slow institution of best practices meant they found rich hunting grounds. This "Second Happy Time" was in many ways the height of the German efforts to disrupt Allied trade with attacks from Canada down the Atlantic Seaboard to the Caribbean and the Gulf of Mexico. In the first half of 1942 in the Americas for the loss of about 25 boats the KM sank over 600 vessels of all sizes and accounted for about 20% of all tonnage sunk by the U-Boats in WW2(3 million GRT of 15). It only ended with attrition of available boats, better SIGINT, better US practices with new technology and weapons, and even the transfer of British warships and patrol aircraft from the UK to North America.
But that was effectively the limit of Nazi Germany's ability to strike at the US.
why then did they even bother with America before they were ready for a full out attack?
I'm going to address this part of the question.
Since almost the start of the war, the US had been supplying Britain (and France before June 1940) with all kinds of material support. From basics like food & fuel to aircaft & destroyers. While the u-boats had dented the supply routes slightly the US had taken a very pro-active stance in protecting the convoys.
USN destroyers regularly escorted UK bound convoys out to the mid-Atlantic and conducted patrols in the Western Atlantyic. This was the Neutrality Patrol which was promoted as ensuring that the Atlantic seaboard of the Americas did not become a battleground. But which in reality was firmly based in ensuring that aid continued to flow to Britain.
In September 1941 the USS Greer dropped depth charges on a German U-boat. In October the USS Kearny dropped depth charges on a u-boat and was hit by a torpedo, but did not sink. Finally, on October 31st 1941 the USN destroyer Reuben James was sunk while escorting a convoy.
Obviously all of this interferred significantly with u-boat operations in the area. The German navy command had requested that Hitler allow operations against the USN in the Atlantic. Hitler declined those requests. When the Japanese attacked the US, Hitler saw an opportunity to attack the USN in the Atlantic whil they were distracted by events in the Pacific.
Finally it does bear pointing out that in the first week of December 1941 the German army was at the gates of Moscow. Although the Soviet winter counter-offensive had just started, there was no appreciation in Berlin that this would be anything more than a spoiling attack. The war in the East appeared as good as won. All that was left was to force Britain to agree to peace terms.