The fact that Madrid has been the capital of Spain since the 16th century is a well-established part of Spanish history. The specific decision to establish Madrid as the capital under Philip II can be found in various historical resources, including:

"The Pursuit of Spanish History" by William Maltby (2016). Here, Maltby discusses Philip II's decision to make Madrid the capital of Spain.

"The Spanish Monarchy and Irish Mercenaries: The Wild Geese in Spain, 1618-68" by R.A. Stradling (1994). This book also talks about the history of Madrid as the capital.

You can also look up this fact in encyclopedias and reliable online resources such as the Encyclopaedia Britannica, or the official tourism website of Spain.
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