It’s time for the U.S. to stiffen its resolve: These thugs are at war with us, we must be at war with them.

Stop and absorb this fact: Jihadi terrorists, one of them an American citizen radicalized at least in part on American soil, attacked a meeting of county officials in a medium-sized American city. Not a highly symbolic target. Not a military installation or a police precinct. Not a contest in which people flaunted their drawings of the Prophet Muhammed. But a holiday luncheon of workaday Americans.

They did this because they — as allies or agents of ISIS — are at war against the United States and the West and see innocent Americans as mortal enemies of Islam, and therefore marked for death.

Tashfeen Malik, one of the murderers, pledged allegiance on her Facebook page to the murderous proto-state that calls itself a caliphate. Still unknown is whether she and her husband Syed Farook were acting on direct orders from Muslim radicals overseas; they might simply have been inspired by videos and missives easy to find online.

For investigators, who now have enough evidence to formally pursue the mass murder as terrorism, the answer matters greatly. For the American public, it is less material.

Either way, the fact is that religiously motivated fanatics, including at least some already in our midst, are eager for opportunities to kill by the dozens and terrorize by the thousands.

The idea grows more horrifying with the understanding that the perpetrators here were a recently married couple with a 6-month-old baby girl. That says all one needs to know about the pitchblack, nihilist hole into which Americans can be dragged, even in a nation that has been known to assimilate Muslims far better than Europe.

So too the fact that Farook knew the co-workers he slayed — their stories, their charms, perhaps even their families.

Blinded by a perverse, all-consuming, theocratic ideology, he deemed fellow residents of the nation that had given great opportunities to his parents, immigrants from Pakistan, enemies, simply because they were American.

Farook’s older brother went one way — serving nobly in the U.S. military. Farook chose to kill innocents for the enemy.

The revelation that this is war means the nation must stiffen its resolve against ISIS targets overseas, ensuring they have no safe harbor.

It means Apple and Google must swiftly change their indefensible policy of selling phones and other devices that cannot be unlocked by police or FBI, even with a judge’s permission. If they don’t come to their senses, Congress must step in.

It means law enforcement must act, as they almost always do, with discipline and focus. Investigators’ seemingly premature release back to the landlord of the shooters’ home yesterday enabled the bizarre spectacle of TV reporters putting their fingerprints on dozens of sensitive documents.

It means ordinary Americans who witness radicalization in progress — including of family members — must inform the authorities.

And it means that, yes, lawmakers must finally snap out of the dangerous delusion that it is responsible to allow psychotic people and suspected terrorists to legally buy weapons.

They are at war with us. Are we at war with them?