 Complete the following phrase:  In the 1920s, Yahya had expanded his power to the north into Tihamah and Asir, but he collided with the rising influence of the Saudi king of Nejd and Hejaz, Abdul Aziz ibn Saud. In the early 1930s, Saudi forces retook much of these gains before withdrawing from some of the area, including the southern Tihamah city of Al Hudaydah. The present-day boundary with Saudi Arabia was established by 20 May 1934 Treaty of Taif, following the 