The Schengen Agreement is a pivotal treaty that was initially signed 39 years ago by five European countries: Germany, France, Belgium, Luxembourg, and the Netherlands. Named after the small town of Schengen in Luxembourg, where it was signed on June 14, 1985, the agreement marked the beginning of a significant process aimed at gradually eliminating border controls between the participating countries. This initiative was intended to facilitate free movement of people and goods across their common borders.

Since the agreement's inception, the number of countries participating in the Schengen Area has steadily increased. The Schengen Agreement initially covered the five founding countries, but over the years, additional nations have joined. Notably, since 1995, several countries have signed the agreement and subsequently implemented the Schengen acquis, which encompasses the body of laws and regulations governing the Schengen Area.

Here's a detailed chronology of countries that joined the Schengen Zone after the original five:

• November 27, 1990: Italy signed the Schengen Agreement.
• June 25, 1991: Spain and Portugal signed the Schengen Agreement.
• November 6, 1992: Greece signed the Schengen Agreement.
• April 28, 1996: Austria signed the Schengen Agreement.
• December 19, 1996: Denmark, Finland, Sweden, Iceland, and Norway signed the Schengen Agreement.
• October 26, 2004: Switzerland signed the Schengen Agreement.

Each of these countries initially signed the Schengen Agreement, and after a period of preparation, they began fully implementing the Schengen acquis. For the other member states within the Schengen Zone, the date of their agreement signing and implementation of the Schengen rules are generally the same, without a separate transition period.