In 1939, British Prime Minister Neville Chamberlain and French Prime Minister Edouard Daladier warned Adolf Hitler that if the Third Reich invaded Poland, a European war would follow.
Both leaders insisted that they meant it. But Hitler thought that after getting away with militarizing the Rhineland, annexing Austria and dismantling Czechoslovakia, the Allied appeasers were once again just bluffing.
England and France declared war two days after Hitler entered Poland.
Once hard-won deterrence is lost, it is almost impossible to restore credibility without terrible costs and danger.
Last week, Russian officials warned the Obama administration about the installation of a new anti-ballistic missile system in Romania and talked of a possible nuclear confrontation that would reduce the host country to "smoking ruins" and "neutralize" any American-sponsored missile system.
Such apocalyptic rhetoric follows months of Russian bullying of nearby neutral Sweden, harassment of U.S. ships and planes, warnings to NATO nations in Europe, and constant threats to the Baltic states and former Soviet republics.
China just warned the United States to keep its ships and planes away from its new artificial island and military base in the Spratly archipelago - plopped down in the middle of the South China Sea to control international sea lanes.
Iranian leaders routinely threaten to close down the key Strait of Hormuz. North Korea and the Islamic State are upping their usual unhinged bombast to new levels - from threatening nuclear strikes on the U.S. homeland to drawing up hit lists of Americans targeted for death.
All the saber-rattling of 2016 is beginning to sound a lot like the boasts and bullying of Fascist Italy, Imperial Japan and Nazi Germany of the 1930s.
But why so much tough talk - and why now?
After the abject pullout from Iraq in 2011 and the subsequent collapse of the country eroded U.S. credibility, after the fake Syrian red lines, the failed reset with Russia, the Benghazi fiasco and the slashing of the military, America has lost its old deterrence.
In a recent interview, President Obama claimed that his Syrian flip-flop was one of his prouder moments, and he disparaged some of our allies (presumably Britain and France among them) as unreliable, glory-hogging freeloaders.
Israel has formed an alliance with some of its longtime enemies in the Persian Gulf based on their shared fears of Iran and their mutual distrust of American commitment. Israelis and Saudi Arabians alike are confused about whether the Obama administration naively appeased Iran with a nuclear deal or deliberately courted it as a new ally.
Japan and South Korea have hinted about going nuclear, prompted by their growing distrust of decades-old American pledges to protect them from neighborhood bullies such as China, North Korea and Russia.
In a recent New York Times Magazine interview, deputy national security adviser and presidential speechwriter Ben Rhodes ridiculed the "Blob" - his derogatory term for the bipartisan Washington foreign policy establishment. He also bragged about deceiving journalists and policy wonks in order to ram through the Iran deal without Senate approval or public support. Mr. Rhodes, who wrote Mr. Obama's mythological "Cairo Speech" and also the infamous Benghazi "talking points," seemed to confirm accusations that this administration has contempt for traditional U.S. foreign policy.
If we know how and when the United States lost its ability to deter enemies and protect friends, why is the world suddenly heating up in the last year of Mr. Obama's presidency?
Recent interviews with the president and his advisers might confirm the impression abroad that the global order is, for a rare moment, up for grabs, as a lame-duck administration retreats from America's role of world leader. And given that there are only eight months left to take advantage of this global void, Russia, China, Iran, North Korea and Islamic terrorists are beginning to believe that the United States will not do anything to stop their aggressions once they change global realities by force.
South Korea, Estonia, Japan, Romania, the Czech Republic, Poland, the Philippines and much of Europe all expect provocations - and fear the U.S. might issue more red lines, deadlines and step-over lines rather than come to their aid.
Aggressors are not sure whether Hillary Clinton, if elected, will govern more like a traditional Democratic president committed to leading the Western alliance. And if Donald Trump were to be elected, no aggressor would know exactly why, when or how he might strike back at them.
Given those uncertainties, it may seem wise in the waning months of 2016 for aggressors to go for broke against the predictable Obama administration before the game is declared over in 2017.
For that reason, the next few months may prove the most dangerous since World War II.
• Victor Davis Hanson is a classicist and historian with the Hoover Institution at Stanford University.