Complex sentence: Robert was originally intended to secure the northern borders of Flanders by his marriage to Gertrude of Holland, but after his brother's death in 1070 he displaced his nephews and became count of Flanders.
Split into simpler sentences: Robert was originally intended to secure the northern borders of Flanders by his marriage to Gertrude of Holland. But after his brother's death in 1070 he displaced his nephews and became count of Flanders.