What countries flag is red with a white cross in the middle?

There are two.

The first is from the country of Switzerland, whose national flag is red with a white cross exactly in the middle.

Note that the Swedish Flag is a square, not a rectangle, and the ends of the cross do not touch the edges. The Swiss Cross, as it is often called by the native population, is a generalization of the coat of arms of canton Schwyz, one of the three founding members of the Swiss confederation back in 1291.

Similarly, the national flag of Denmark, is also red, but the white cross in the middle is off-centre and reaches the edges of the flag. Like most national flags, it is rectangle.

The name of the Danish flag is the Dannebrog, meaning ‘the flag of the Danes’ or ‘the red flag.’ According to legend, the Dannebrog fell from heaven during a battle in Estonia.

For more on flags of all countries visit Flagpedia.net.