On 13 September 1940, the Italian Tenth Army crossed the border from the Italian colony of Libya into Egypt, where British troops were protecting the Suez Canal. The Italian invasion carried through to Sidi Barrani, approximately 95 km inside Egypt. The Italians then began to entrench. At this time there were only 30,000 British available to defend against 250,000 Italian troops. The Italian decision to halt the advance is generally credited to them being unaware of the British strength, and the activity of British naval forces operating in the Mediterranean to interfere with Italian supply lines. There were Royal Navy seaports at Alexandria, Haifa, and Port Said. Following the halt of the Italian Tenth Army, the British used the Western Desert Force's Jock columns to harass their lines in Egypt.