Why does the Maven command “mvn sonar:sonar” work without any plugin configuration in my “pom.xml”?

The reason is that the Sonar Maven Plugin is hosted at the Codehaus Mojo project and benefits from the groupId “org.codehaus.mojo”. This allows to use the shortcut “sonar:sonar” instead of “org.codehaus.mojo:sonar-maven-plugin::sonar” (see the section “Configuring Maven to Search for Plugins” of the Maven documentation)