Yes, it is. We do, of course, have our differences, but we are fundamentally one nation.
Americans do differ based on things like race and ethnicity and religious values. Our "culture wars" sometimes make it seem as if we are separate nations. But at the core of things, we are not. To see how true this is, just look at the European Union. There, you have people in Germany not wanting to help people in Greece because they don't think they really have much in common. Here, people grumble about taxes in general, but it's not as if people in New York, for example, think that they shouldn't be helping people in Louisiana recover from Hurricane Katrina. We share deep common bonds of history and a common commitment to the basic ideas of our government. We are one nation.
Yes, we are one nation under God! Although at times, it may seem we are a divided nation, especially when listening to our modern day politicians who cannot agree on anything these days.
The United States has always served as the "melting pot" and the "land of opportunity." We were formed under these ideals, and they still hold true over 200 years since the signing of the Declaration of Indepencence. We assimilate the best of all cultures in our multicultural nation. We always have and always will do this! We may have our differences at times, but yes, we are still one nation under God with liberty and justice for all.