Roosevelt does this by gradually comparing previous historical precedents of American history with the current moment. He starts his demonstrative enumeration with 1789, a very symbolic year since it is the year when the Constitution of the US came to have legal force (it is also the year of the French Revolution which is hardly a coincidence). Roosevelt delivered this speech before Congress on January 6th, 1941. Context is paramount here: at that point in time, WWII was well under way, and Nazi Germany and the Axis powers were winning victory after victory. France had already fallen (thence the date of 1789 is definitely not a coincidence), so had Norway, Denmark, Belgium, The Netherlands, Belgium, Poland, etc. the only major Western European country that still resisted Nazi Germany was the United Kingdom and Churchill was desperately in need of both material and moral support from America. The Nazi army seemed unstoppable and the Blitz was raging over London. Roosevelt shows that due to technological advances the world was much smaller than the previous antebellum world in which the great oceanic distances between America and Europe/Asia protect the former from invasion or political turmoil. Also, the viciousness and the hostility of the Axis made it inevitable that once they had secured dominance of Europe and its colonies, they would come for the USA. Because of such dangerous and imminent threats, this moment was unique in history and Roosevelt provided factual evidence of that in its speech.
