10 Small Decisions That Changed the Course of History and 10 Small Mistakes That Made Everything Worse

In 1928, Alexander Fleming left a Petri dish out in the open before going on vacation, and upon his return, he noticed that the mold had killed the bacteria in it. A more organized scientist would have thrown it away; Fleming, however, conducted research, identified Penicillium notatum, and thus ushered in the era of antibiotics.

In 1911, Winston Churchill switched the Royal Navy from coal to oil—a risky gamble, given that Britain had virtually no oil reserves of its own. Motor-powered ships were faster and easier to refuel, which gave Britain a decisive advantage during World War I and quietly helped shape a century of geopolitics in the Middle East.

While observing a wine press used to crush grapes, Gutenberg realized that the same mechanism could be used to print inked characters onto paper. This innovative idea gave rise to movable-type printing, which in turn paved the way for the Protestant Reformation, the Scientific Revolution, and mass literacy.

Charles Darwin almost didn’t board the Beagle. The captain almost refused to take him on board because of the shape of his nose, and Darwin’s father disapproved of the voyage. He set sail anyway, and that five-year voyage led to the observations that resulted in the publication of On the Origin of Species.

On September 26, 1983, Soviet early-warning systems detected five American nuclear missiles approaching. The officer on duty, Stanislav Petrov, concluded that it was a malfunction, reasoning that a genuine first strike would not be limited to just five missiles—and he was right.

After years of unsuccessful experiments, Michael Faraday wound two coils of metal wire around an iron ring in 1831 and noticed a spark when he passed a current through it. He realized that this was electromagnetic induction, the principle that has formed the basis of all electric generators ever since.

The success of the landing depended on the ability to convince Germany that the invasion would take place in the Pas-de-Calais rather than in Normandy. Operation Fortitude, with its fake army groups, inflatable tanks, and fake radio communications, sustained this belief long enough to prevent German Panzer reserves from reaching the beaches in time.

In the 1840s, Ignaz Semmelweis noticed that the mortality rate at a Viennese maternity hospital decreased when doctors washed their hands with chlorinated lime before delivering babies. He implemented this practice despite being mocked, and it worked.

Seward advised Lincoln not to issue the Emancipation Proclamation following a series of Union defeats, warning him that it would appear to be an act of desperation. Lincoln held off on the proclamation for two months, until the Battle of Antietam, and this choice of timing transformed the proclamation from a plea for help into a show of strength.

When, in 1947, Bedouin shepherds brought ancient manuscripts to an antiquities dealer in Bethlehem, researcher John Trever recognized their importance and photographed them before they were sold off piece by piece. His swift action helped preserve the Dead Sea Scrolls, including the oldest known manuscripts of the Hebrew Bible.

Here are 10 small mistakes that made matters worse.

On June 28, 1914, Archduke Franz Ferdinand’s motorcade took a wrong turn in Sarajevo and came to a stop in front of Gavrilo Princip, one of the assassins that day, who had already abandoned his plan and gone into a grocery store. Princip fired two shots, and within six weeks, this assassination had triggered World War I.

The Swedish warship Vasa was the most powerful ship in the Baltic Sea when it was launched in 1628, but it sank less than a kilometer away. The crew had opened the lower gunports to fire a celebratory volley; these gunports were barely one meter above the waterline, so water rushed in and the Vasa sank before everyone’s eyes in Stockholm Harbor.

When Napoleon placed his brother on the Spanish throne in 1808, he believed the country would submit just as the rest of Europe had. Instead, he faced six years of civilian resistance so unique that it gave the world the word “guerrilla,” thereby depleting the resources he would have needed in Russia.

On December 7, 1941, cryptographers decoded a message warning of an imminent Japanese attack, and General Marshall sent a warning to Pearl Harbor via commercial telegram due to radio interference. Western Union delivered it by bicycle courier several hours after the bombing began.

In 1999, the Mars Climate Orbiter probe was lost because one team of engineers was using the metric system while another was using the imperial system. This discrepancy caused the probe to enter the Martian atmosphere at the wrong angle, bringing a $327 million mission to an end due to a simple unit conversion error.

The safety test that caused the Chernobyl disaster in 1986 had been postponed by nearly a day and assigned to a night shift team that was not familiar with the procedure. The reactor was placed in an unstable, low-power state, but the test was carried out anyway, causing the explosion that contaminated approximately 150,000 square kilometers in Europe.

On April 14, 1912, the Titanic had received at least six warnings about the presence of ice and was still sailing at nearly full speed when it struck the iceberg. The decision not to slow down was not due to ignorance; it resulted from time pressures and an overconfidence in a ship that its builders had described as unsinkable.

Napoleon launched his Russian campaign in late June 1812, with very little time left before winter set in, and the Russians refused to engage in battle, retreating and burning their supplies as they went. Moscow had been evacuated and set ablaze; he waited five weeks for a surrender that never came, then retreated across that same frozen countryside.

By the end of May 1940, German panzers had cornered the British Expeditionary Force in Dunkirk when Hitler ordered a three-day truce. This respite allowed Britain to evacuate more than 330,000 Allied soldiers, who formed the core of the army that would return to Western Europe four years later.

The Bay of Pigs operation failed largely because Kennedy called off, on the eve of the landing, the airstrikes intended to destroy what remained of Castro’s air force, in the hope of being able to deny any involvement. Castro’s planes sank two supply ships and pinned the brigade of exiles down on the beach, and the operation collapsed within three days.