I think there are some very important lessons to be learned from this.
1. The power of the POTUS to act unilaterally on military and foreign policy (probably policy in general, now that I think about it) really should be checked. Trump's blunder has proven that the POTUS can't always be trusted to act in accordance with America's interests or basic human decency.
2. Other Western countries can't rely on the US alone to contribute significant ground forces in military operations. Had the UK, France, etc. also had a significant presence in Northeastern Syria, Trump's decision to withdraw wouldn't have had the consequences it did.
3. Refusing to stand up to or call out somebody just because your interests happen to align in some areas can and will come back to bite you. Trump's "allies" (read: sycophants) in Congress helped make the mess we're in.
4. Never rely on somebody who's proven to be as mercurial, reckless, irresponsible and inconsistent as Trump has.
On a related note, how do you feel about mental competence tests for politicians? Because after this, I really think they're a good idea.