After reading Megamorphs 3: Elfangor's Secret a few weeks ago, I was thinking about places Visser Four should have travelled to instead of what we're given: Agincourt, Trafalgar, Dellaware, Marathon, Alternate United States of America Post World War One, and the Alternate Second World War.
I think that, if Visser Four wanted to really hurt America and its dominence, he should have gone back in time and killed Christopher Columbus (the explorer, not the director of the first two Harry Potter movies), thus stopping him from [ACCIDENTALLY] discovering America.
He should also have also gone back to Greece and turned the tide of the Persian wars. Imagine how much culture would have been lost if he killed one of the Athenian generals (I know we learned his name, I just can't remember...). Also, the battles between Rome and Carthage--such a thing could have obliterated the Roman Empire.
To name a few more: Waterloo--the battle between France and England and Prussia that literally changed the world-- any of the victories of the Entent during the First World War (I'm sure England and its allies were the Entent...I can't remember. I just know the term Allies switches in WW1 and WW2), the pop culture explosion in America, the forming of the Beatles (hey, think about where music would be without our wonderful Beatles), etc.