Error assembling WAR: webxml attribute is required (or pre-existing WEB-INF/web.xml maven on war

You might come across this error when building a web application on maven using the command

mvn war:war

This would be caused if maven is not aware of where the web.xml file is located.
If you follow the standard maven directory structure as

src/main/java
src/main/resources
src/main/webapp

You would have to copy the contents of WebContent [WEB-INF AND META-INF] to src/main/webapp folder – there maven would get it as expected.