You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Warsaw_Pact

Context:
On 12 March 1999, the Czech Republic, Hungary, and Poland joined NATO; Bulgaria, Estonia, Latvia, Lithuania, Romania, and Slovakia joined in March 2004; Albania joined on 1 April 2009.

Question:
In which year did Slovakia join NATO? 

2004