Short answer: for a little bit.

The long answer, as usual, is somewhat complicated.

German immigration was high at the outbreak of WWI, peaking in the 1880s. They flooded the Midwest and left their mark on the region to this day. But it is important to note that by the time the First World War came around the heyday of German immigration had passed. Most German-Americans were now second generation, and identified as American as much as anybody. That being said, German reservists clamored to go back and defend the fatherland, while concerned letters were sent to relatives still living there and "relief parties" organized to send money. So while there was pro-German sentiment among German-Americans at the outset of World War I, it was nothing unusual for any immigrant group at the time, or really at any time in history. And of course as the war went on the actions of the German Empire alienated most of its American supporters. The sinking of the Lusitania, the Rape of Belgium, and the Zimmermann Note turned German-Americans against their own country, and ironically most other Americans against German-Americans. In fact, over 2,000 of them were interned in camps much like the Japanese in WWII, and laws were passed to restrict the imagined possibility of a German revolt. In essence, to most German-Americans the "Fatherland" was as much a tangible place as an idea; something to hold onto and to identify with. Few German-Americans would identify as anything other than American, let alone die for anything other than America. There was certainly no active initiative among German-Americans to undermine the war effort, at most pro-German sentiment was mainly concern for relatives abroad.

In terms of the Second World War, most German immigrants fled Germany for a good reason. Hitler deported many Jews, and many Jews and political opponents of Hitler fled on their own, including theoretical physicist Albert Einstein. As such, while pro-German sentiment was more organized and more active (pro-Nazi organizations like the German-American Bund actually received a respectable amount of members) before the Second World War than the first, it was quite a minority in comparison. Keep in mind, German immigration to the United States took a nosedive during the Great Depression and the leadup to WWII, and by then many German-Americans were third or fourth generation, making their allegiance to Germany (especially the newfangled and alien Third Reich, which had replaced the German Empire they remembered) was symbolic at best. By the time America entered the war it was united against Germany. Admiral Chester W. Nimitz and Supreme Commander Dwight D. Eisenhower were both of German ancestry, as were many American soldiers.