On the sixteenth anniversary of 9/11, it’s time for a reminder of the lessons we should have learned — and that we have now unlearned. On that horrific day, on a bright sunshine-riddled morning, Islamist terrorists flew two passenger airplanes into the World Trade Center towers, collapsing them both and murdering nearly 3,000 Americans; flew another passenger airliner into the Pentagon; and were only prevented from flying a fourth into the White House or Capitol by the bravery of passengers who forced the crash of the flight in a field in Pennsylvania.

America responded with the full brunt of her wrath. For a brief moment in time, we were united against a common enemy: we reduced Afghanistan’s ruling regime to powerlessness, then turned and reduced the Saddam Hussein regime to smoldering ash. We increased bureaucracy and security at our airports. We poured money into building our military. Some of this was necessary; some of it wasn’t. But we knew one thing: we wouldn’t allow ourselves to be hit again. We wouldn’t be caught asleep at the wheel again.

We had learned our lessons.

But now those lessons have been largely forgotten. Here were some of the lessons we should have learned.

1. Global Retreat Is Not A Strategy. The Clinton Administration foreign policy of quasi-isolationism, combined with occasional human rights-driven interventionism, was a formula for failure. After Osama Bin Laden’s Al Qaeda bombed the American embassies in Kenya and Tanzania, the United States did virtually nothing; again, the U.S. did virtually nothing after Bin Laden bombed the USS Cole. Bin Laden saw the United States as a paper tiger. That prompted him to strike, thinking that he would get away with it.

The Bush Administration’s aggressive response took Al Qaeda by surprise. According to James Mitchell, the man who questioned Al Qaeda mastermind Khaled Sheikh Mohammed, KSM stated, “How was I supposed to know that cowboy George Bush would announce he wanted us ‘dead or alive’ and then invade Afghanistan to hunt us down?” Mitchell writes, “KSM explained that if the United States had treated 9/11 like a law enforcement matter, he would have had time to launch a second wave of attacks.”

But the blowback from the Iraq War led the Obama Administration to imitate the foreign policy of the Clinton Administration, and in many ways, has led the Trump Administration to do the same.

2. Money Doesn’t Buy Off Islamists. Neither Does Friendliness. For a decade, the Clinton Administration reached out to the Palestinian government with cash, pressure on our ally Israel, and symbolic moves to legitimize the regime. The response: on 9/11, the Palestinians danced in the streets and celebrated as Americans leapt to their deaths from burning buildings.

One of the great myths purveyed by Bin Laden was that the United States had been overtly hostile toward Muslims around the world. That’s nonsense. We used the power of America’s military to stop the invasion of Kuwait; we sided with the mujahedeen in Afghanistan against the Soviets; we activated NATO to prevent a genocide against Croatian Muslims in former Yugoslavia. Friendliness toward the Muslim world does not matter to Islamists, who seek only the domination of a religious caliphate.

Yet the Obama Administration sought friendship with the Iranian government, essentially handing them a nuclear program and control of a vast swath of the Middle East.

3. Immigration Matters. President Trump is right to look to immigration vetting as a serious problem in handling of the terror threat. All 19 of the hijackers arrived on visas, either student, tourist, or business. Several of them overstayed their visas. While Trump worries mostly about immigration via the southern border, an estimated 40% of illegal immigration occurs through legal entry and then visa overstay. Our porous southern border is a problem. A bigger problem is the government’s utter failure to vet people entering the country generally and then their complete unwillingness to follow up on those who overstay their visas. While the Left complains about Trump’s refugee stay, the fact is that the government ought to be deeply concerned about those who enter the country from Islamist-rich regions. Europe is finding that out the hard way through increased crime and terrorism.

4. Major Terrorist Attacks Require Sponsor States. This was a widely-accepted truism in the aftermath of the 9/11 attacks — that major attacks require planning, coordination and resources that demand a home base. That’s why the Bush Administration took out the Taliban, and looked (wrongly) to Iraq as a sponsor state of terrorism (they should have looked more closely at Iran). That logic led the Bush Administration to think that replacement of regimes with more democratic institutions would naturally effectuate less terror; the Obama Administration, in its bizarre argument in favor of the so-called Arab Spring, felt the same way. That was a problem with the implementation of the theory, not the theory itself. Major terrorist operations do require state sponsors — that’s why ISIS’ territorial integrity matters to its sponsorship of major terror operations overseas.

5. America Has Real Enemies. It’s tempting for the United States to look inward for threats to its citizens — we’re the freest, most prosperous country in human history. As Obama noted, Al Qaeda was never an existential threat to us, nor is ISIS. But when it comes to threats to American citizens, the first duty of the government is to prevent those threats and stop those who would perpetrate them. We should be unified in that effort, not divided for political reasons.

Many of the lessons we learned on 9/11 have faded with time. We seem to be back where we started — in the miasma of Clintonian isolationism, although tempered by a stronger anti-terror mechanism abroad. That’s why it’s imperative that we once again remember what happened on 9/11 — and what we must do to stop the next 9/11.