"on the back foot" , what does it mean?

"Italian Prime Minister Silvio Berlusconi is on the back foot again. His lawyers say an audio tape which a magazine today claimed has the voices of Berlusconi with a prostitute are false and are part of an ongoing investigation. His attorney also warns he will take legal action against anyone who distributes those recordings."

I don't get it, what does "on the back foot" mean?

Thanks in advance.