From the 15th century onward, all Holy Roman Emperors but one were Austrian archdukes and with the acquisition of the Bohemian and Hungarian crown lands in 1526, the Habsburg hereditary lands became the centre of a major European power.
The passage describes how the Habsburgs, starting from the 15th century, held the position of Holy Roman Emperor but one was an Austrian archduke.  Their expansion in 1526 solidified their control over Bohemian and Hungarian territories, making them a major European power.