We had to use clues and evidence to work out reasons that would be likely or unlikely.  We discovered that there were several reasons why they invaded and we then decided if these reasons were push or pull factors.  

For instance, a pull factor was that they were interested in the metals available in Britain. A push factor was the flooding in their homelands that had ruined their crops.

We then watched and evaluated two different video clips about why they invaded to decide which video clip was more reliable and historically accurate.