By 1609, a great European power recognised the independence of rebellious sections of the Netherlands, a small part of its empire, having not been able to put the revolt down. To explain this, one must look at both Spanish weaknesses and Dutch strengths which prevented Spain from being able to commit sufficient resources to the Netherlands. However, we must ultimately look to Spanish rather than Dutch factors to explain why Spain recognised the independence of the Netherlands in 1609.
Various Dutch factors contributed to the Netherlands being able to resist Spanish forces, and led to the revolt surviving until 1609 (a pre-condition of gaining its independence) as well as making the revolt a sufficient obstacle for Spain that Philip III decided to recognise the United Provinces' independence in 1609.
The Dutch received a significant degree of foreign support until they gained their independence. The treaty of Nonsuch granted the Netherlands 6500 soldiers and paid for them from 1585, and, earlier on, Casimir's army may have made a contribution.
However, Casimir proved more interested in fighting local Catholics than the Spanish, and as a result made little lasting impact on the course of the revolt. The Earl of Leicester also proved incompetent in handling military affairs. Moreover, neither contribution was of sufficient extent to prevent Spain from accomplishing their goals, and did not make a lasting positive contribution to the Netherlands until 1609 or a lasting negative impact on Spain. It is therefore not possible to explain why the Netherlands gained their independence in terms of the foreign aid that they received.
The Northern provinces benefited from a boom in economic prosperity and enjoyed very effective regenerative policies. Due to persecution in the provinces conquered by Spinola and Parma, many Calvinists had moved to the remaining rebel provinces after...