The American Legion has released a booklet called "A Strategy to Address Illegal Immigration in the United States" [1]. I haven't read it, but Sonia Scherr of the Southern Poverty Law Center apparently has and offers "American Legion Immigration Report Replete With Falsehoods" [2]. Based only on what she says, it appears the the AL made a few mistakes and left themselves open to groups like the SPLC - which is indirectly linked to the Mexican government - raising issues. They should have foreseen that would happen and subjected it to stringent review; that might be happening now [1].

That said, Scherr's "debunking" a) rests parts of its claims simply on how data is interpreted, and b) contains errors of its own. Consider this:

The Legion's Claim: Thousands of immigrants from countries with terrorist connections have been caught trying to enter the United States illegally. This claim is immediately followed by several paragraphs about illegal crossing of the Mexican border, suggesting that those "from countries with known terrorist connections" are entering the U.S. via Mexico.

The Truth: A 2006 report by the Nixon Center, which appeared in the peer-reviewed Terrorism and Political Violence journal, found that the suggestion that terrorists were entering the U.S. through Mexico, while frightening, was entirely untrue: "Despite media alarms about terrorists concealed in the illegal traffic crossing the Mexican border, not a single subject entered from Mexico."

Scherr's "truth" doesn't rebut anything in the AL's claim, and AL's claim is indisputable: it's backed by dozens of news reports as well as government reports. See this category and this. And, the Nixon Center report apparently only dealt with a subset of known terrorists; it's a pay-only report so it's not clear whether it included those with terrorist links or those suspected of being terrorists (link). And, people with terrorist links have indeed crossed over the border or been involved in smuggling operations. And, of course, terrorists aren't exactly known for announcing their presence ahead of time. Overall, it's difficult to believe that there haven't been some terrorists crossing over either porous border, and it's also extremely foolish to base belief in the opposite on one study or to assume that terrorists don't want to cross over the porous borders. It's also indicative of the level the SPLC is willing to sink to in order to support massive illegal activity.

Regarding their "truth" about immigrants bringing exotic diseases into the U.S., see "The ADL's definition of "hate" and "anti-immigrant" can't be trusted. (ALIPAC, others)".

[1] The booklet was apparently available here, but that says it's being updated. Also, the AL needs to pay more attention to search engines; my first search for their booklet showed Alternet and related sites on the first page and nothing from their organization.
[2] alternet.org/immigration/95695 Note that I've been banned from Alternet or I'd leave a link to this post there.