Cannot execute mojo: tree. It requires a project with an existing pom.xml, but the build is not using one

When run below command :

[root@www 
~]# mvn dependency:tree

Result:

[INFO] 
Scanning for projects...

[INFO] 
Searching repository for plugin with prefix: 'dependency'.

[INFO] 
------------------------------------------------------------------------

[INFO] 
Building Maven Default Project

[INFO] 
   task-segment: [dependency:tree]

[INFO] 
------------------------------------------------------------------------

[INFO] 
------------------------------------------------------------------------

[ERROR] 
BUILD ERROR

[INFO] 
------------------------------------------------------------------------

[INFO] 
Cannot execute mojo: tree. It requires a project with an existing pom.xml, but 
the build is not using one.

[INFO] 
------------------------------------------------------------------------

[INFO] 
For more information, run Maven with the -e switch

[INFO] 
------------------------------------------------------------------------

[INFO] 
Total time: < 1 second

[INFO] 
Finished at: Wed Jun 26 02:38:40 EDT 2013

[INFO] 
Final Memory: 3M/61M

[INFO] 
------------------------------------------------------------------------

Try with the -e option:

[root@www ~]# mvn -e dependency:tree

Result:

+ Error stacktraces are turned on.

[INFO] 
Scanning for projects...

[INFO] 
Searching repository for plugin with prefix: 'dependency'.

[INFO] 
------------------------------------------------------------------------

[INFO] 
Building Maven Default Project

[INFO] 
   task-segment: [dependency:tree]

[INFO] 
------------------------------------------------------------------------

[INFO] 
------------------------------------------------------------------------

[ERROR] 
BUILD ERROR

[INFO] 
------------------------------------------------------------------------

[INFO] Cannot 
execute mojo: tree. It requires a project with an existing pom.xml, but the 
build is not using one.

[INFO] 
------------------------------------------------------------------------

[INFO] 
Trace

org.apache.maven.lifecycle.LifecycleExecutionException: 
Cannot execute mojo: tree. It requires a project with an existing pom.xml, but 
the build is not using one.

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:719)

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeStandaloneGoal(DefaultLifecycleExecutor.java:569)

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:539)

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:387)

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:348)

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:180)

  
      at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:328)

  
      at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:138)

  
      at org.apache.maven.cli.MavenCli.main(MavenCli.java:362)

  
      at 
org.apache.maven.cli.compat.CompatibleMain.main(CompatibleMain.java:60)

  
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

  
      at 
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

  
      at 
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

  
      at java.lang.reflect.Method.invoke(Method.java:597)

  
      at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)

  
      at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)

  
      at 
org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)

  
      at org.codehaus.classworlds.Launcher.main(Launcher.java:375)

Caused 
by: org.apache.maven.plugin.MojoExecutionException: Cannot execute mojo: tree. 
It requires a project with an existing pom.xml, but the build is not using 
one.

  
      at 
org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:414)

  
      at 
org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:694)

  
      ... 17 more

[INFO] 
------------------------------------------------------------------------

[INFO] 
Total time: 1 second

[INFO] 
Finished at: Wed Jun 26 02:38:57 EDT 2013

[INFO] 
Final Memory: 3M/61M

[INFO] 
------------------------------------------------------------------------

Normally this error happens, when you try to run the mvn dependency:tree  command from a location, where it does not find a POM.
mvn dependency:tree  is used to see/verify the dependency between jars used in a particular project.

So, the solution is :
navigate to your project directory and where you have the pom.xml file for the project you want to verify the jar dependencies:
For example, like

# cd /opt/cas-workspace/local-cas

Thats the location of my workspace, containing my projects pom file.

[root@www local-cas]# ls -l

total 
20

-rw-r--r-- 
1 root root 8795 Jun 25 11:33 pom.xml

drwxr-xr-x 
3 root root 4096 Jun  5 07:33 src

drwxr-xr-x 
5 root root 4096 Jun 25 11:57 target

So, now you can run the mvn dependency:tree command.

References:
http://maven.apache.org/plugins/maven-dependency-plugin/usage.html
http://maven.apache.org/plugins/maven-dependency-plugin/examples/resolving-conflicts-using-the-dependency-tree.html