Complex sentence: Shortly before the outbreak of World War II, Hans Oster called Dohnanyi into the Abwehr of the Oberkommando der Wehrmacht, led by Wilhelm Canaris, which quite quickly became a hub of resistance activity against Adolf Hitler.
Split into simpler sentences: Shortly before the outbreak of World War II, Hans Oster called Dohnanyi into the Abwehr of the Oberkommando der Wehrmacht. Led by Wilhelm Canaris, it quite quickly became a hub of resistance activity against Adolf Hitler.