That's what it was. The NSA was reverse proxying Google. The legit explanation (given the domain name) is probably they wanted to use reCAPTCHA, but block all

That's what it was. The NSA was reverse proxying Google. The legit explanation (given the domain name) is probably they wanted to use reCAPTCHA, but block all non-NSA hosts with a firewall or something. This is not great, because the NSA expanded its attack surface to all of google.com. The more conspiracy explanation is that this is actually a phishing page set up, and due to a misconfiguration it's exposed under captcha.nsa.gov, but Occam's Razor should apply here.