Problem with `include` command

Question

I have prepared a cover page and the first chapter of my dissertation. Both of these are in different tex files.

I read that in order to have everything in a master file you need to use the include command. It does not work for me so far.

Here is the minimal example.

% cover_page.tex
\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   %%%%%%%
%%%%%%%%%%%%%%%%%%%%%%         TITLE, AUTHOR FOLLOWS            %%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\title{\textbf{{\huge Proposal}} \\[8pt] Essays on ...}
\author{by Me}
\date{}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%    DOCUMENT BEGINS    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\begin{document}
\maketitle
\thispagestyle{empty} % no number in the first page

\vspace{3cm}
\begin{center}
Advisor: 
\end{center}

\vspace{2.5cm}


\begin{minipage}[c]{14.5cm}
\centering Proposal 
\end{minipage}


 \vspace{3cm}
 \begin{minipage}[c]{14.5cm}
 \centering Department \\
  University \\
\end{minipage}


\vspace{2.5cm}
\begin{center}
\date{A month}
\end{center}

\newpage
\thispagestyle{empty}

\begin{center}
\begin{tabular}{lllll}
 Advisor & \hspace{2cm} & \hspace{1cm} & Me & \hspace{2cm} \\[25pt] 
 \cline{1-2}\cline{4-5}
 \textit{Signature} & & & \textit{Signature} &  \\[25pt] 
 \cline{1-2}\cline{4-5}
 \textit{Date}  & & & \textit{Date} &  \\ 
 \end{tabular} 
 \end{center}

 \end{document}

Now let us move on chapter_1.tex

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\title{My title}
\author{PSK}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%    DOCUMENT BEGINS    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}
\maketitle
\thispagestyle{empty} % no number in the first page

\newpage

Some stuff here 

\end{document}

And last, but not least the master file:

\documentclass{article}

\begin{document}
\include{cover_page.tex}
\include{chapter_1.tex}

\end{document}