OK I will bite:
13 But we do not want you to be uninformed, brethren, about those who are asleep, so that you will not grieve as do the rest who have no hope.
So the topic of the following verses is Paul's explanation about those who die before the second coming.
14 For if we believe that Jesus died and rose again, even so God will bring with Him those who have fallen asleep in Jesus.
15 For this we say to you by the word of the Lord, that we who are alive and remain until the coming of the Lord, will not precede those who have fallen asleep.
16 For the Lord Himself will descend from heaven with a shout, with the voice of the archangel and with the trumpet of God, and the dead in Christ will rise first.
17 Then we who are alive and remain will be caught up together with them in the clouds to meet the Lord in the air, and so we shall always be with the Lord.
18 Therefore comfort one another with these words.
(1Th*4:13-18*NAU)
These verses are about as clear as it gets. The resurrection will happen first, then the rapture. If Paul believed in two raptures, one pre-trib and another one later, he would not have been able to make these statements. He would have had to say something like "some of you will be with the Lord before the resurrection", and the whole argument would have been meaningless. We are to comfort each other when someone dies... they will get to see the Lord before the living.
The pre-trib rapture theory apparently originated with Darby and the Plymouth Brethren (they called it the "secret rapture"). I have not been able to find any references that pre-date Darby.