Complex sentence: Their Tizard Mission to the US showed how far ahead they were in some of the technologies, particularly the magnetron, and it was decided to send a US team to Britain to help coordinate the efforts of the two Allies.
Split into simpler sentences: Their Tizard Mission to the US showed how far ahead they were in some of the technologies, particularly the magnetron. The US decided to send a team to Britain to help coordinate the efforts of the two Allies.