After years of uncertainty and upheaval allowed ISIS militants to gain a foothold in the country, the U.S. has begun carrying out airstrikes to try and oust them.

Source: http://edition.cnn.com/2016/08/04/africa/libya-chaos-in-graphics/index.html

Don't you think that there is "that" omitted in the sentence above (…upheaval that allowed…). I was thought that when relative pronoun “that” is a subject, you cannot omit the relative pronoun. Or the first clause is not a relative clause?