The largest nation in the world by far is Russia, which is then followed by Canada, the United States, and China, in which Canada, the United States, and China all are very close to being the same size in land mass. Depending upon the economist, the USA took over as having the largest GDP in the world from Great Britain perhaps as soon as 1880, and certainly no later than the commencement of WW I. No country, even today, is close to the USA's GDP but because of China's high growth rate and its greater population (the most in the world), in contrast to the USA's much lower growth rate and much smaller population, projections show that China should surpass the USA as the largest country in the world by GDP in 2019 or thereabouts. While there may be doubt as to what year this will occur, there is little doubt that it is inevitable that it will occur.
The USA prides itself on being #1, and there is a very straightforward way for the USA to maintain its status as #1 and that is through a concerted program of sustained population growth. To get an idea of how under populated and how empty the United States is, just simply get in your automobile and drive around the country and you will notice acres and acres, miles upon miles, of empty land, underutilized and under occupied. To get a further perspective, businessinsider.com estimates that half of our population resides in just 146 counties out of a total county list in the USA of over 3000. Additionally, worldatlas.com states that South Korea has a population density of 1273.5 persons per square mile, as compared to our meager79.55/sq mile.
The United States is gifted with its great capacity for food production in which we are the largest exporter of food in the world. The USA also has an abundance of energy sources, from coal, to hydroelectric, to natural gas, to oil (in all of its forms), to renewable, and also nuclear energy to not only sustain its growth but to expand it. The USA is a mature country in which the infrastructure that we need to communicate, trade, travel, and educate ourselves is firmly in place.
What America lacks at this point is population growth which currently is less than 0.75% per year, hasn't been above 1% since 2000, and hasn't been above 2% since 1950. Our population growth isn't close to being what is needed for the USA to sustain being the most powerful country in the world in the sense of GDP and power. Even with China taking over as the #1 nation in the world in regards to GDP, that will not equate to their becoming the most powerful or the most influential nation in the world, however, it will inexorably change the mindset of the other major nations in the world and at such time as the world transfers and acknowledges that there is a new king, it will dim the light of the USA and its influence.
If you believe that what China brings to the world, are exactly what the world needs and desires, than we have little or nothing to worry about. On the other hand, if you believe that the USA represents the last best hope of mankind, it is high time; in fact, well beyond high time, that we make an immediate and massive increase in our population, by incentives, by immigration, by all means necessary. Remember, the best way to destroy your enemy is to make him your friend, and that can be readily done by residing, participating, and living the American way in America.