In 216 BC, at a place in southern Italy called Cannae, Hannibal destroyed the largest army Rome had ever raised, tens of thousands killed in a single afternoon. With the capital nearly defenseless, he chose not to march on it. That restraint let a broken Rome survive, and the Republic that lived went on to build the Western world.

It almost went the other way. Hannibal's own cavalry commander, Maharbal, urged him to take the horsemen and ride through the night, swearing they'd dine in the enemy capital within five days. Hannibal said he needed time to think. The reply Maharbal threw back has echoed for twenty-two centuries: you know how to win a victory, but you don't know how to use one.

What if he'd listened, and turned the horses north at dawn?

This week we hit Control Z on the victory Hannibal couldn't use, follow the march to the gates of Rome, and the very different world it leaves behind.

In December 2000, the Supreme Court stopped Florida's recount and let George W. Bush's 537-vote lead stand. Those votes, out of nearly six million cast, decided Florida, and Florida decided the presidency.

It almost went the other way. Seven of the nine justices agreed the count was broken. They split on the fix. Five voted to stop. Four voted to send it back, set one standard, and finish. The man in the middle was Anthony Kennedy, who agreed the recount was broken and then voted to end it instead of fixing it.

What if he'd changed his mind on that one question, and said count it instead of stop it?

This week we hit Control Z on the vote that didn't switch, follow the recount to its finish, and the very different country it leaves behind.

In October 1555, a dying Charles the Fifth split the largest empire in Europe between two heirs. Spain, Italy, the Netherlands, and the silver of the New World went to his son Philip. The imperial crown, plus Austria and the lands to the east, went to his brother Ferdinand. The most powerful man in Christendom looked at everything he held and cut it in half.

He almost didn't. Years earlier, Charles had schemed to keep it whole, to put every crown on his son's head and make Philip both King of Spain and Holy Roman Emperor. The plan died in a family fight. The empire stayed divided.

What if it hadn't? What if one devout, distant, Spanish king had inherited the whole thing, and then tried to rule German princes who'd never accept him?

This week we hit Control Z on the division, and follow the unbroken empire as it breaks everything it touches.

In 1948, the Supreme Court ruled that Hollywood's five biggest studios had been running an illegal monopoly. People remember the case as the government ordering the studios to sell their theaters. That's not what happened. The Court banned the worst practices, sent the theater question back to a lower court for a fresh look, and left the door open.

Howard Hughes walked through it in the wrong direction. He volunteered to split RKO's studio from its theaters before the lower court even ruled. Every other studio watched him do it and followed.

The system that had controlled American moviemaking since the 1920s collapsed in under a decade. What grew in its place changed everything.

This week we hit Control Z on Hughes's signature, and follow what happens when the first domino refuses to fall.

On October 20th, 1980, Ronald Reagan wrote the president of the air traffic controllers' union a letter calling their working conditions "deplorable" and promising his administration would fix them. PATCO endorsed him three days later. It was the first time the union had ever backed a Republican.

Ten months into his presidency, 11,400 controllers walked off the job. Reagan gave them forty-eight hours to come back. When the deadline passed, he fired every single one of them, broke the union, and dared the FAA to keep the planes flying with less than a third of its workforce.

The planes kept flying. The consequences took longer to arrive.

This week we hit Control Z on the forty-eight-hour deadline, and follow what happens when the president who promised to fix the system decides not to destroy it.

On a computer, Control Z is the undo button. When you press it, whatever you just typed gets reversed and allows you to make a different decision.

What if you could press control Z on some of the biggest decisions in history?

In October of 2016, Bob Iger was 48 hours from buying Twitter for $15 billion. Both boards had approved. The lawyers were drafting. Goldman Sachs was working the weekend. Then Iger went through the user data one more time, read his own notifications, and couldn't sleep. Sunday morning he typed an email to his board with the subject line "cold feet," called Jack Dorsey, and killed the deal. Twenty-eight days later, Donald Trump won the presidency.

Fourteen months after that, Disney bought 21st Century Fox instead, and Disney+ followed.

This week we hit Control Z on the Sunday morning phone call, and follow what happens when the man who runs Disney decides to own the platform that hosts the president.

In November of 1095, a sixty-year-old French pope stood in a field outside the cathedral of Clermont and gave the most consequential speech of the Middle Ages. Eight months earlier, ambassadors from the Byzantine emperor had asked him for a few thousand professional knights to help fight the Seljuk Turks. Standard contract work. Urban II sat with the request through the summer, then walked outside on November 27th and offered something nobody had ever heard before. Full remission of sins for everyone who took the cross. The crowd shouted back, "Deus vult." God wills it.

They asked for soldiers. He gave them a holy war.

This week we hit Control Z on the speech that launched the Crusades, and follow what happens when one man writes the smaller answer.