Complex sentence: Emerging from a intelligence sharing agreement as part of the Atlantic Charter and the 1943 BRUSA Agreement, it was formally signed in March 1946 by the United Kingdom and the United States and later extended to encompass the three Commonwealth realms of Canada, Australia and New Zealand.
Split into simpler sentences: Emerging from an intelligence sharing agreement as part of the Atlantic Charter, it was renewed with the 1943 BRUSA Agreement before being formally enacted in March 1946 by the United Kingdom and the United States. In the following years, it was extended to encompass the three Commonwealth realms of Canada, Australia and New Zealand.