How about Red Chinese undercover agents and soldiers?
Why would they come in over the border when they can be inserted as embassy employees with diplomatic immunity? Or given third country covers via say, genuine Peruvian passports with non-Chinese names? They could come in on business. As students. As journalists. The legal immigration system is also a sieve. They can marry American citizens or green card holders and get official papers. This is an open society. We don't have armies of secret police to follow people around and monitor who they talk to. Do you really think immigration officials - of whom there aren't even enough to seal the border - keep track of who comes into the country legally, where they are and what they're doing? The 9/11 hijackers all came in on regular tourist visas.
The best way to do undercover work is to do it legally. Chinese nationals can get green cards and join the military. They can get undergrad or grad degrees here and join defense-related companies. None of this involves crossing the southern border illegally. The real problem with an open southern border is that as long as this country pays much higher wages than 80% of the rest of the world, illegals will stream in without end, continuing to drive vast swathes of low end American labor to welfare. Which, in the long run, is debilitating to the welfare recipients, the country's economy and crime rates. And a huge waste of human talent.