Complex sentence: According to Piketty, that drop results from the creation of income tax, and the strong growth of its progressivity after the war, which thwarted the dynamics of estate accumulation, by reducing the available savings for the wealthiest.
Split into simpler sentences: According to Piketty, that drop results from the creation of income tax, and the strong growth of its progressivity after the war. This broke the dynamics of estate accumulation, by reducing the available savings for the wealthiest.