Here's how I see it:

1. Britain is a world powet and settled in the Americas (among other regions)
2. Americans gained independence from the British, and cut academic ties
3. Years go on and American culture evolves separately from the UK
4. America becomes dominant military and economic force and English becomes essential for international business
5. American English as codified gets distributed worldwide

The English used today is very similar grammatically to the language used 200 years ago, so despite the Wild West days of the US, I think education has done a pretty good job of keeping the language stable. We obviously get new phrases and idioms as culture evolves, but grammar has remained pretty consistent

Here are the historical major events

• ~1600 - British as a world power start to make English dominant
• starting ~1400, the great vowel shift occurred - English spelling started to get standardized in this period
• 1066 - Norman conquest - lots of French loan words are introduced to old English
• ~800 - Viking invasions begin, bringing lots of Norwegian words and simplification of Old English grammar
• ~450 - Saxon invasion and Germanification of the older pre-English language

Then look at English from the 1700s to today, over ~300 years, we've had very few changes in grammar. I guess thee and thou finally fell out of favor, but there's really not many changes, and that's about as long as the period between any of those major events.