Well, since it has been brought back, I'll add my two cents. First, I find it strange that no one has mentioned that France also declared war on Germany, and paid for it, before they got any help as a nation as far as I know. Pretty much all of the Commonwealth nations also declared war when England did. That included Canada, who entered according to a Wiki post I read, in Sep 39.
From what I recall of history discussed immediately after the war, and what I learned in high school, is that indeed many in the USA in the 30s did not want to be involved in war anywhere, including Europe and Asia. We were selling scrap metal to Japan until some time just before Japan attacked. Some think that may have made it easier for them to attack. Japan had taken over much of the Pacific that suited them, especially where raw material was, such as Dutch holdings.
At any rate, Americans didn't want to go to war due to remembrances of WWI, and being embroiled in their own depression. Also, not a well remembered fact is that there were a lot of communists, although they began to loose favor. They were lobbying against Germany, as Stalin didn't much like them and felt threatened by them. There was also a surprising amount of sentiment for the Germans, due to a lot of Americans having ancestry back to Germany. Also, they had brought themselves out of their depression. Those were complex times.
Even so, many felt sympathy for England, and her stand against Germany. Roosevelt and others wanted to help, but not violate neutrality. That was what lend-lease was all about. We weren't giving them war materials, we were lending and/or leasing things. Rather a thin line, but it worked for a while. Germany was getting tired of it, and might have begun heavily preying on our convoys themselves sooner had not Japan pushed us into the war when they did. I seem to recall there were some instances even so.
When Japan attacked us, they did us great harm at the beginning of the war. They destroyed and damaged a lot of our warships. As an aside, there have been many conspiracy theories. Conspiracy theories pop up all the time, but most just don't seem to get validated, but they sound neat to some people, so they get repeated a lot. If our leaders knew what was going to happen, why in the world would anyone think they would take such a chance? The Japanese had us on our knees for a while, and could have taken Hawaii without too much trouble, probably meaning the rest of our fleet. As to the earlier mention of it strange our best ships were not there? Our best ships were the battleships. The Navy hadn't strongly committed to carriers yet, and in fact, the "Old Navy" still considered battleship command the top choice. They happened to be out on training. Period. They were not considered the 'best' ships at the time.
As others have pointed out, when America entered the war, it had access to a fairly large manufacturing base, and a lot of raw material available in all the Americas. That made it easier for them to supply not only themselves, but all their allies. Doing so prevented them from being reduced to the only ones fighting the war. Even so, we had a lot to do to be able to fight a global war. It wasn't something we started doing on December 8, 1941. Had it not been for lend-lease, it would have probably taken longer. It helped us build up industry, and train our armed forces. Not well known, is that we did have State Guard units, even though they were very poorly trained and equiped. But it was a start.
So, as to Monroe's original question, what if England didn't go to war? I have to also say, who knows? War had already been started by Japan, Germany, and Italy. At some point, the USA and other North and South American countries would have had to make a decision to cowtow to the Axis, or fight them. Even when Americans have been most inclined not to fight over whatever issues, there is a point beyond which they will not go.
I suspect it is the same with England and Canada, and even other Commonwealth nations. Somebody correct me if I am wrong.
So I suspect that England would eventually have had to fight or be totally subdued by Germany and Italy. Would they have done so in time to have suffered less than they did? I don't know. Glad it didn't happen that way, but I can imagine a possibility such that those who wished to remain free would have had to relocate and set up a government in exile. Like some other countries had to do. Would have been a totally different war.
One final thing regarding English and other countries' debt. At the end of WWII, I don't think Churchhill or Truman had a lot of trust for Stalin. That may be part of the reason it was easy to begin aid to England and Europe right after the war. Within about 3 years, that aid was formalized as the Marshall plan. Beside just being a good thing to do (which we always like to think we do), it was a good way to build up Europe against the Communists. But even they were offered a chance to participate in that. Reasonably, how much debt could we expect them to take on and still begin to recover? And England wasn't the only one to repay debt.
Some of the smaller countries continued to repay the WWI debt after WWII as I recall. So getting a 21 cents on the dollar deal and bragging about that? Well, we like to brag about things that perhaps we shouldn't, so I'm not going to throw stones. Besides, on my mother's side, I have English heritage. ;-)
As I said, just my two cents.