Did God ever really help the Hebrews in battle? Except for the Exodus story, im having serious doubts. 

The Hebrews never seem to win against any of the bigger enemies which would be seriously impressive, because they are outnumbered by many factors. Looking at the figures of their victories they seem to have won because they outnumbered their opponent or simply due to better battle tactics. 

After all if God really wanted to show his power why didn't he ever defeat Rome, Greece, Egypt or Babylon?

Is this a fair comment?
Here's a list of several events just off the top of my head:


capture/destruction of Jericho
Othniel
Gideon's 300 men against the Midianites
Samson's final crush of the Philistines
Elijah and the prophets of Baal


As to why God didn't save them from some oppressors, one of the myriad instances of a single phrase in Judges shows why:


  Now the sons of Israel again did evil in the sight of the LORD, so that the LORD gave them into the hands of the {nation} for {number} of years*




*note "nation" and "number" can be substituted for any oppressor and duration