Complex sentence: On this date Marconi Electronic Systems and British Aerospace merged to form BAE SYSTEMS; their respective subsidiaries in the United States, Marconi North America and British Aerospace North America, were similarly merged to form BAE Systems North America.
Split into simpler sentences: On this date Marconi Electronic Systems (MES) and British Aerospace (BAe) merged to form BAE Systems. Their respective subsidiaries in the United States (US), Marconi North America and British Aerospace North America, were similarly merged to form BAE Systems North America.