Over the past few years, Poland has frequently encountered this situation, and there have been financial penalties to pay.
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Here some interesting info about the current state of borders in the EU:
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There are repeated temporary reintroductions, for example due to the COVID-19 pandemic or during international events such as the Olympic Games and the European Football Championship in 2024. In addition, some EU member states have effectively reintroduced permanent border controls in recent years. Austria has continuously controlled its borders with Hungary and Slovenia since 2015 to "limit migration."
The European Court of Justice (ECJ) ruled in 2022 that this practice violates the Schengen Borders Code. Germany reintroduced border controls with Austria for the same purpose in 2015, which have been continuously extended to this day. In a January 2024 ruling, the Munich Administrative Court, citing ECJ case law, found that these controls also do not comply with the European legal requirements of the Schengen Borders Code and are therefore unlawful.....
ecchr.eu/fall/kontrollen-und-zurueckweisungen-an-den-deutschen-grenzen/
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The reintroduction of border controls can be reviewed by the European Court of Justice (ECJ). However, such a procedure cannot be initiated by individuals or civil society organizations. This would actually be the responsibility of the European Commission, which, as the so-called "guardian of the treaties," has a supervisory function.
However, the Commission's previous practice has shown that, for political reasons, it largely refrains from initiating infringement proceedings against member states in the area of ​​migration and border control. However, in addition to the European Commission, other EU member states can also initiate such controls. The Schengen Borders Code provides for a special "consultation procedure" (Article 27a).....