The united states was motivated to begin pursuing an imperialist agenda at the end of the nineteenth century because the country

was producing more manufactured goods than its population could use.

Explanation:

American imperialism was a policy applied by the government with the purpose to extend political, cultural, and economic influence of America overseas.  

In the late nineteenth century, age of the 'New Imperialism' began in America. In this age, the two important factors of imperialism were Industry and Trade. In the words of Albert Beveridge, the need for pursuing imperialism and expansion of industry was necessary because the goods produced by American industry were more than what were consumed by its population. He, further stated, that because there was overproduction of goods they had to expand the American industry making the trade industry of the world theirs.  

So, the correct answer is because of more production of goods than consumed by the population, American imperialism was needed.

is there a year set? the question is incomplete