Secretary of State John Kerry told a painful truth on Sunday, admitting that President Obama’s “red line” fiasco in Syria “cost us significantly” by leading other nations to see America as weak.

Obama drew the line in August 2012 — as a way to avoid getting involved in Syria’s civil war without having to actually justify that restraint. He did so by saying he would intervene if the government did the truly awful, by using chemical weapons.

“We have been very clear to the Assad regime … that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” he said, also calling “chem” use a “game-changer.”

It was universally seen as a threat of massive consequences for Bashar al-Assad if he crossed the line. But when Assad did launch chemical attacks a year later, Obama stalled.

First he tried to get allies on board with a campaign of airstrikes. Britain declined, but France said yes — yet Obama then asked Congress to OK the bombing. Congress signaled reluctance to “buy in” — and then Obama accepted a diplomatic lifeline from Moscow to negotiate a deal for Syria to (supposedly) turn over all its chem munitions.

The president had blinked at making good on his own threat. Around the globe, US allies and enemies were on notice that America might not live up to its word.

It’s no coincidence that Russia took control of Crimea within the year, and later intervened decisively in Syria to save Assad. Nor that Iran was able to virtually dictate the terms of its nuclear deal with Team Obama.

Kerry on Sunday fell back on two excuses. First he argued that it was a “misperception” that Washington had been weak, since Assad did give up (many of) his chems. But even he had to admit that “it doesn’t matter. It cost. Perception can often just be the reality.”

He also faulted Congress for not immediately OK’ing the use of force once the president asked — without noting that Obama, in the year after he drew the red line, never laid a bit of groundwork with Congress (or with America’s allies) for enforcing it.

If you issue a threat, then don’t even prepare to make good on it, all you’ve really made is … an empty bluff.

Now graphic scenes like this one have become all too common in Syria: