Diplomacy.

After 9/11, when the world weighed an invasion of Afghanistan, America could have skipped the invasion, taken the Al Qaeda leadership the Taliban offered up, and continued to seek O/UBL. A forensic investigation and specific arrests, extradition, trials, and convictions would have been much better than a disastrous 20 year war that accomplished two things: enriching military contractors and the impoverishment of a central Asian nation.

Diplomacy.

Deposing Saddam Hussein with the same type of pressure that, later, led to the ousters of Hosni Mubarak, Ben Ali, and Bashar al Assad. Some might say that 2003 created the pretext for the Arab Spring. I'd counter that time and tide created the conditions. Operation Iraqi Freedom was a pipe dream and an extension on the GWoT piggy bank.

Diplomacy.

Building a better, more sustainable future demands a move away from fossil fuels. Making driving, urban sprawl, warfare, agribiz, and Amazon packages into a socially toxic soup of ideas would have done wonders for green initiatives. Instead a turn away from the largest industries of the time was — and still is — regarded as heresy.