In 1580 King Philip II of Spain seized the Portuguese throne. The illegitimate claimant, Dom Antonio, Prior of Crato, fled to Protestant England, and from there plotted the overthrow of the King of Spain. The presence of Dom Antonio added to the existing tension between England and Spain, which came to a head in 1588, when Spain launched the Armada against England. Under Drake, the Armada was repulsed, but the outcome was inconclusive. War dragged on and by 1594 had become costly and unpopular, without any end in sight.
David Katz argues that the Portuguese community in England were heavily involved in politics because of the disputed Portuguese throne. They were initially on the side of Dom Antonio, but Dom Antonio was a waning asset. Nor did he pay his agents adequately, whereas the Spanish paid generously for any information. A lot of the Portuguese became double agents, with documents revealing that information was sold several times over. David argues that Lopez was one of those tempted by the money of King Philip II.
Was the Queen's doctor, Lopez found guilty of conspiring to murder Elizabeth I because of anti-Semitism in 16th century England? Cecil Roth, author of 'A History of Jews in England', written during the Second World War, thought so. Many historians since have echoed this judgement. But David Katz felt that the whole book, written as it was by a Jewish historian, showed only the positive contribution of the Jewish community to English life, and downplayed or excluded any negative evidence.
Katz, himself a Jewish historian, wanted to set the record straight and write a warts and all history of Jews in England. He believed that original documents hadn't been extensively consulted, and that the international connections of the Jewish community hadn't been taken into account.
Starting with the Calendar of State Papers, a printed summary of English government documents, he identified the crucial sources. He then referred to the originals, and on reading them became convinced of Lopez's guilt. All the evidence pointed to it.