When you fall back on something it means you depend on it when other things have failed.
Steve was finding it difficult to make the repayments and had no savings to fall back on.
Rebecca knew that securing a mortgage would be a problem, but she always had her parents to fall back on.
If I can't get a job straight away, I've always got my experience as a teacher to fall back on.
Don't confuse it with
To fall out with someone means to argue or disagree about something.
Dave fell out with Bob over his broken CD player.
Many nations hold an annual celebration of their Armed Forces. South Korea's commemoration falls on October 1st, the date in 1950 when armed forces broke through the line of latitude known as the '38th parallel' during the Korean war.