WWII is always one that's kind of dogged me. After Pearl Harbor, there wasn't much that can be done, but there's much to criticize about our economic policy towards Japan beforehand. I've always dodged the question by saying if we hadn't stupidly aggresed against the Central Powers in World War I, there wouldn't be a World War II (Say what you will about German U-Boats, Wilson had clearly picked his side long before and wanted an excuse to "Make the world safe for Democracy."
You could argue if you like that violence wasn't justified in the Revolutionary War, but the reality is that WE were the secessionists. England used lethal violence to make them stay in the country. Did not Abe Lincoln do the same to the Confederate States?
I really don't see how you can justify Lincoln. Ft. Sumter is weak when you consider the South offering to pay for the fort, but the US refusing to acknowledge CSA and rearming the fort, as well as constantly telling the South they had to return.