The answer to this can be found in Judges, Chapter 6.  

God had allowed the Israelites to be oppressed by their enemies because they had not obeyed God and his laws.  But then God relented and decided to help the Israelites once again.  He chose to use Gideon as the instrument by which to defeat the Midianites.  

God sent an angel to Gideon.  The angel came to Gideon while Gideon was threshing wheat.  Gideon was doing this in a winepress so that the Midianites would not be able to see what he was doing and take his grain.  This showed how badly the Israelites had been humbled and oppressed.