Right from the start, the shrewd Sir William Cecil served as Elizabeth's chief Secretary of State. In 1571, Elizabeth named him Lord Burleigh, moved him to the position of Lord Treasurer, and replaced him as Secretary of State with the more cutthroat but nevertheless loyal Francis Walsingham. The talents of Burleigh and Walsingham, and Elizabeth's ability to work productively with them, were perhaps the most crucial strengths of Elizabeth's rule. And even Elizabeth's more minor advisors, including Gresham, a financial advisor as stingy as Elizabeth, were talented and valuable.
Elizabeth and Burleigh did not always get along, however. Although she had already signed Mary Queen of Scots's death-warrant, when Burleigh pushed through the execution ahead of schedule to "spare" the queen some emotional hardship, Elizabeth became outraged and banished him from her presence. But while she constantly argued with him and sometimes abused his good nature, in the end she said of him, "No Prince in Europe had such a counselor."
Francis Walsingham, Elizabeth's second key advisor, was quite different from Burleigh, but also extremely capable. A fanatical Protestant, Walsingham did not have nearly the same personal admiration for Elizabeth that Lord Burleigh had. Nonetheless, Walsingham had a powerful sense of duty and was so utterly devoted to England as a nation that he served Elizabeth with supreme loyalty. Although they did not see eye-to-eye on all matters, Elizabeth replaced Burleigh with Walsingham as Secretary of State in 1572, making him her most crucial advisor. Walsingham specialized in organizing spies. He built an intelligence network that kept Elizabeth aware of many of the most secret plots and machinations throughout both England and Europe. Although Elizabeth found his fanatical Protestantism distasteful, this flaw was more than outweighed by Walsingham's ability as a master of espionage.
Where Walsingham's skills failed him, however, was in his attempted orchestration of a marriage between Elizabeth and the Duke of Anjou. Afraid that Anjou might marry Mary Queen of Scots instead of Elizabeth, he considered this a Protestant duty: he wanted to create an English-French alliance that would protect the Protestant Netherlands from Philip II's bloody rule. Walsingham even managed to get Leicester on his side in the matter. Yet Elizabeth continued to fool everyone on the question of marriage, and pretended to consider Anjou's proposal before finding grounds to reject it.
Around 1585, it was Walsingham's spy network that picked up on Mary Queen of Scots' secret return to Scotland. Ingeniously, Walsingham intercepted the messages she was sending hidden in beer barrels, deciphered them, and unearthed the Babington Plot against Elizabeth. This information proved Mary Queen of Scots' treason, which allowed Elizabeth to sign a death warrant against her. When Leicester reported to Walsingham that Elizabeth had received a letter from the condemned Mary that made her cry, Walsingham got together with Burleigh to push the actual execution through before the Queen had a chance to change her mind.
Elizabeth was an especially intelligent ruler. However, many intelligent rulers try to do all the work without assistance and surround themselves with incompetent people who will offer mere sycophantic praise rather than true advice. Elizabeth was not prey to this failing, and recognized the need to surround herself with able and shrewd advisors who would be willing to disagree with her when she was wrong. Her willingness and ability to make use of talented advisors, particularly Lord Burleigh (William Cecil) and Francis Walsingham, was one of the factors that most contributed to the "greatness" of her reign. While she worked herself very hard, she demanded even more sustained effort from her advisors, several of whom she worked to the point of ill health.