Complex sentence: He transitioned Cold War policy from détente to rollback, by escalating an arms race with the USSR while engaging in talks with Gorbachev, which culminated in the INF Treaty, shrinking both countries' nuclear arsenals.
Split into simpler sentences: He transitioned Cold War policy from détente to rollback by escalating an arms race with the USSR while engaging in talks with Gorbachev. The talks culminated in the INF Treaty, which shrunk both countries' nuclear arsenals.