During the course of my work, I visited a little-known Spanish territory sitting on the continent of Africa just across from Gibraltar called Ceuta, or, if you are in Morocco, Sebta. It’s one of three Spanish protectorates on North African soil surrounded by Morocco ever since the sixteenth century -- and has been a sore spot between the two nations ever since. But once you walked about twenty yards and stepped across an unseen line on the ground into Ceuta you were technically in Spain… in Europe, the EU.

That meant that after catching a quick ferry ride across the Mediterranean you were literally on the European continent and free to go anywhere in the EU without passport checks. The challenge was getting from Moroccan land across that unseen line into Ceuta. But it was not hard to do.

I crossed through the Ceuta border post often, going back and forth from my home base in Spain. Since it’s my job to notice details, I noticed that the border was less than secure -- on both sides. Sure, both countries have the requisite “3-Bs” for securing a position -- boys, bullets, and barriers. Winding its way between Ceuta, Spain, and Morocco, there was a double fence topped with razor wire, reminding me of the U.S.–Mexican border. There were guard dogs, spotlights, riot vans with their windshields covered in steel mesh. There were men on both sides armed with submachine guns; there were metal detectors, there were multiple steel gates, and it all made the border appear well protected.

But in reality it wasn’t.

The metal detectors on either side were turned off nearly every time I went through. A well-dressed bad guy with nice suitcases could bring weapons or anything else across without much trouble at all -- just by giving off the good citizen appearance. But there was something even simpler, something worse.

Assume for a moment that Libya has surface-to-air missiles, MANPADS, floating around in the hands of radical militant Islamist bad guys (which they do). Then assume that a group of those bad guys wants to strike Western targets, killing as many as possible (which they do). How do they do it? Sail across the Mediterranean Sea? No, they’ll be boarded by a NATO warship, arrested, and thrown in jail. Take a ferry out of Benghazi? No, they’ll be stopped at the port in Europe.

First, they head southwest, crossing into Algeria, where they sell or transfer the weapon to al-Murabitoun -- the main al-Qa’ida jihadi group in Algeria -- who then continue the trek southwest, crossing into Mauritania at the porous border near Chegga. Once inside, they make their way to Nouadhibou, an Atlantic port city just a few thousand yards south of Western Sahara -- which is essentially Morocco. They hire a simple wooden fishing boat, push out into the Atlantic, head north a few miles, and beach it on the Moroccan Atlantic coastline of Western Sahara. Fishermen are everywhere along that area, so it’s nothing out of the ordinary. The terrorist, having coordinated a vehicle with sympathizers in Morocco, heads north up through the modern highway to Agadir, turns right for Marrakech, hangs a left toward Casablanca, and then Rabat, all the way to... you guessed it, to the northernmost point of Ceuta, Spain... yet still on the African continent.

But you can’t just walk through with a five-foot-long surface-to-air missile -- that’ll draw attention whether the metal detectors are on or not. But again, that crossing point has water on the eastern side. They go to a Moroccan tourist shop, buy an inflatable pool raft, wait until dark, and float the weapon twenty yards north from one side to the other, passing the border station to the Spanish side. Kick their way back to shore, knife-sink the raft, hail one of the dozens of cabs that make a U-turn at Martinez Catana Avenue looking for customers, and the bad guys are technically in Spain -- the EU. Once inside, they catch one of the daily ferries leaving the marina at Puerto de Ceuta for mainland Spain, and there’ll be no further checks on the other side of the ferry ride -- they, their weapon, and their evil intent are in Europe. It’s like catching a bus to go from Madrid to Barcelona, or from Canton, Ohio, to Chicago -- no checks.

“Once on the mainland, they are able to go anywhere with anything, crossing borders unchallenged all across Europe, to airports in Spain, Italy, France, crossing through the Chunnel into England and “armed with a shoulder-launched missile capable of blowing a slow, fat passenger jet completely out of the sky. How many Ceutas are there? How many holes, how many threats around the world? Lots -- and they are simple to exploit.

From my experience, the simple plans work. Box cutters took down three jet airplanes on 9/11; simple trucks with explosives destroyed our embassy and Marine barracks in Lebanon in the 1980s, and the Murrah Federal Building in Oklahoma City was taken out by a rental truck parked out front in 1995. Simple works.

Bad guys could shut down mighty LAX -- Los Angeles International Airport -- or any airport in the United States with a spray bottle filled with ammonia nitrate. A bad guy need only crush up the small white balls out of the one-use instant cold packs that you can buy at almost any CVS or Walgreens pharmacy, or buy a sack of fertilizer and then mix either with water. Then walk through the airport and spray it on the floor, call in a bomb threat, and the K9s will indicate bombs everywhere. The authorities would tear the place to the ground trying to find bombs that don’t exist -- simple works. Get that mixture into the wax used to polish the floors and that airport will be razed.

Bad guys with half a brain can walk through airport security, head to any Hudson News snack store, and buy all the supplies needed to make a deadly blowgun, a functioning frag grenade out of a coffee thermos, an incendiary carry-on suitcase, and even a functioning shotgun just using Axe body spray, lithium batteries, a hair dryer, a handful of quarters, and a few other items. Simple works.

The sniper attacks in the Washington, D.C., area in October 2002 were carried out by two guys whose collective IQ was probably no larger than their shoe sizes, yet they took a rifle, hid in the trunk of their Chevy Caprice, and shot people from a hole bored out next to the license plate. These two moral midgets managed to effectively shut down the nation’s capital. It was the sort of operation that, if deployed across the country in teams, would lock the nation in fits for weeks, shut down commerce in major cities, tie up law enforcement, and cause significant economic damage... for the cost of a few rounds of ammunition.

I hope the good guys realize these weaknesses, because the bad guys do and are always looking for more -- though Tuesday's attacks in Brussels suggest otherwisw. As the late, great British Prime Minister Margaret Thatcher said long ago, the bad guys only have to be right once, but the good guys have to be right all the time.

Jamie Smith is a decorated former CIA officer, author of “Gray Work: Confessions of an American Paramilitary Spy” (WmMorrow/HarperCollins 2015), frequent commentator for FOX News, CNN, MSNBC, former advisor to the Chair of US House Intelligence Terrorism, HUMINT, Analysis, and Counterintelligence Subcommittee, advisor to GRAY | Solutions and holds a doctorate in law.