By Aaron Kliegman\u00a0 Too often we as a society frame immigration as a social, cultural, or economic issue. It\u2019s understandable: We normally discuss immigration in relation to the law, national identity, or jobs. But fundamentally, immigration is an issue of national security \u2014 just as choosing who you let into your home is an issue of personal and familial security. We were reminded of this basic yet crucial fact earlier this week, when federal authorities said they caught two suspected terrorists crossing the southern border. Specifically, US Customs and Border Protection (CBP) revealed Monday that, in recent months, the agency arrested two Yemeni men for illegally entering the US from Mexico. Agents busted the men, ages 33 and 26, near the Calexico Port of Entry in California in separate incidents on Jan. 29 and March 30. Here\u2019s where national security really comes into play: Authorities then discovered by checking their records that both men were on not only a \u201cno-fly\u201d list but also a government watchlist for people suspected of terrorism. One reportedly had a cellphone SIM card hidden in his shoe. According to the FBI, the watchlist includes people who are \u201cknown to be or reasonably suspected of being involved in terrorist activities.\u201d Two of them were just trying to enter our national home illegally. And they\u2019re not the only ones. Yikes. CBP\u2019s announcement came amid a surge in the number of migrants seeking entry in the US. Which begs the question: How many seek to do us harm, and how many have crossed the border successfully? We just don\u2019t know. There\u2019s not enough publicly available data. True, few people who entered the US illegally have been convicted of planning a terrorist attack on American soil, and even fewer have been injured or killed by terrorist attacks. But that doesn\u2019t refute the commonsensical point that a relatively open and vulnerable southern border is an appealing option to hurt \u201cthe Great Satan\u201d for those who wish us harm. Indeed, our enemies would be morons not to exploit this vulnerability \u2014 America\u2019s literal underbelly. Plus, when we look at illegal immigration as a matter of national security, terrorism isn\u2019t the only threat. Of perhaps greater concern is espionage \u2014 the prospect of adversarial countries such as Iran sending agents to spy rather than attack. Just in February, US Border Patrol caught 11 Iranians crossing the southern border from Mexico into Arizona. The Iranians may be ordinary migrants with nothing to hide. But it wouldn\u2019t be surprising if one or more were on a more nefarious mission than just entering the US illegally \u2014 which, by the way, is illegal in and of itself. In fact, Iran has deeply entrenched itself in Latin America through its chief proxy, Lebanese Hezbollah. Both Iran and Hezbollah are deeply involved in drug trafficking and a host of other criminal activities across the region. Add the fact that Iran and Hezbollah have plotted to attack Americans and others on American soil in the past, and it becomes clear that this threat cannot be ignored. Critics will say all this is paranoid, even conspiratorial thinking. It\u2019s not a coincidence that these are the same voices fighting serious efforts to secure our borders more effectively. Indeed, they were probably happy to learn that, one day after announcing the apprehension of the two Yemeni men, CBP took down its press release. Presumably, if federal authorities stop more suspected terrorists trying to cross the border, we won\u2019t hear about it. What a relief: Now we don\u2019t need to worry about the problem. We can just close our eyes and ignore it \u2014 and, like magic, the threat will surely go away. Because there\u2019s absolutely no way imaginable that terrorists or foreign governments would ever exploit our southern border to attack or spy on us. After all, the more open our borders, the more benign we are, and the more the rest of the world likes us. That\u2019s how it works, right?