Why America wouldn't fight before Pearl Harbor is simple.
By the end of the 1930s, Americans experience of the twentieth century has been brutal. Brief periods of economic boom now seem an illusion. And the Great Depression, that they're still feeling, has been accompanied by the worst environmental disaster, the Dust Bowl, in the country's history. They've tried to make a better society with Prohibition and have only made organised crime bigger. When Americans involve themselves abroad, it's either been shamefully as an imperialist oppressor, as in the Philippines, or they've been horrifically exposed to killing on an industrial scale, as in the First World War. Americans are in no mood for a fight. Back in 1898, when a ship of theirs, the Maine was attacked, it was all the excuse America needed to wage war on Spain. (And in 1964, America is even willing to fake an attack on a US vessel to start the Vietnam war). But in 1937, when the Japanese bomb the USS Panay, America accepts their explanation that it was an accident (it wasn't) and back off.
Americans watch and do nothing as Mussolini invades Ethiopia, and as Hitler's Blitzkrieg destroys Poland, Czechoslovakia, Denmark, Norway, Holland, Belgium and France, it's understandable why even the Nazi appeasing British Prime Minister, Neville Chamberlain, says:
It is always best and safest to count on nothing from the Americans but words.
Not even President Roosevelt can shift American popular opinion to come to Britain's aid in its hour of need. But many of those who hate their former colonial masters come to admire the bravery of the pilots that win the Battle of Britain against the Luftwaffe: Though it's still not enough to declare war on Nazi Germany.
So behind the scenes Roosevelt tries to protect British shipping and secures them the money and the weapons to continue fighting. Publicly, he prepares his country for a war they don't want.
Pearl Harbor changes everything as surely as 9/11 will in the next century.