The Battle of Tabuk

It was called Jai shalu al-Rusrah the difficult Jai sh and the Quran references this indirectly in the Quran. They left towards Syria in the blazing summer and they were tested severely so much so that it is mentioned that two or more people were rationed one date per day. It's basically late july or maybe early august which would be the rajab of the ninth year of the hijra. Why did razwata tabuk take place? I have not found a satisfactory answer after teaching syrah many many times and just glossing over this I have come to a conclusion that ishallah will share today.