Kanwardeep Singh ji
You question is based on 3 oversimplifications.
1. According to US Census 2000 the Chinese represent the largest ethnic group in the US at approximately 20 percent. Does this mean that Chinese Americans should partner politically with the Peoples' Republic of China?
2. 15 percent of a population does not represent a majority. The reference you gave shows German Americans to be 17.1 percent of white Americans (white being a racial category). Therefore their percentage is smaller for the total population of the US.
3. And.. I don't know who these "Germans" are you are talking about. First-generation-anything-immigrants are never a political majority and cannot vote until they become citizens. Therefore they cannot influence US policies including who to side with in times of war.

Right?
There is no ethnic majority in the US, though Chinese represent a plurality. A majority requires more than 50 percent.
The history of German migration in the US goes back to the early 1600's. The individuals who followed, followed in the 1700's, 1800's, and 1900's. They settled in states on the Atlantic coast, predominately Pennsylvania and New Jersey. The migrated from there to the west and mid-west. States with heavy German-American populations in the early 1800's included Texas, California, Washington, Pennsylvania, New Jersey, Ohio, and western New York state. Later migration was to places in the upper Mid-West like Michigan, various others.
By the time World War II broke out these people thought of themselves as German-Americans, and as such they felt a common bond with the principles of democracy. Democracy is something that Germany had a problem with historically, but now stands in the forefront as a champion of democratic principles.
In fact they, the immigrants, were leaving Germany first because of religious oppression (1600's and 1700's) and later (1800's) for economic opportunity. When they got here they must have found what they were looking for.
I don't want to tell the whole story of German immigration to the United States. Maybe it would take less time and effort if I were simply to be sarcastic and say something like. Well they were really imbeciles and should have banded together and rebelled against British influences, overthrown the US government, sided with Hitler, and today they would be in charge of the show. That would be hilarious wouldn't it?
But we really should be able to figure out that the Germans who immigrated here were really not coming from "Germany" at all. The Germans, like most European immigrants were coming from "pinds." They were coming from regions that were religiously and economically very different. They came from places like the Palatine, Saxony, Hanover. Their loyalty was not to a Germany which was then nothing more than the league of Hanseatic states.
We also should be able to figure out that Hitler was an obnoxious character. So why should German Americans align with him -- unless of course they were also part and parcel of his morbid and pathological agenda?
During World War II, some German Americans were sent to detainment camps, others went to war. Some German Americans worked to support U-boat incursions into US harbors, some died in naval battles at sea as part of the American navy. Some German Americans, in particular the German Catholic community, knew of Hitler's systematic extermination of the Jews, and asked Roosevelt to intervene before America entered World War II.
At that time Americans had no taste for war, were isolationists, had the view that Europe should dig itself out of its own mess. Once war broke out of course there were feelings of fear and concern -- of having to turn guns on Germans. In the end they did.
The question of why the US sided with England has to be answered on its own stock of historical facts.