The primary purpose behind Roosevelt's words in this excerpt is to

This is the missing excerpt:

"I cannot prophesy. I cannot tell you when or where the United Nations are going to strike next in Europe. But we are going to strike-and strike hard." - President Roosevelt's State of the Union address.

The primary purpose behind Roosevelt’s words in this excerpt is to convince the public of America's strength

He infers that America has the strength to strike and strike hard in Europe. This presents the America as a formidable foe to its citizens.