Tell Maven to download your dependencies by using the dependency plugin like so:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<version>3.1.2</version>
<executions>
<execution>
<id>copy-dependencies</id>
<phase>package</phase>
<goals>
<goal>copy-dependencies</goal>
</goals>
<configuration>
<outputDirectory>${project.build.directory}/lib</outputDirectory>
<overWriteReleases>false</overWriteReleases>
<overWriteSnapshots>false</overWriteSnapshots>
<overWriteIfNewer>true</overWriteIfNewer>
</configuration>
</execution>
</executions>
</plugin>
After that you have the jars in the target/lib directory. Now you can add this directory to your assembly.
I had the same problem with Eclipse v3.7 (Indigo) and m2eclipse as my Maven plugin. The error was easily solved by explicitly stating the execution phase within the plugin definition.
So my pom looks like this:
<!-- language: xml -->
<project>
...
<build>
...
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>buildnumber-maven-plugin</artifactId>
<version>1.0</version>
<configuration>
<timestampFormat>yyyy-MM-dd_HH-mm-ss</timestampFormat>
</configuration>
<executions>
<execution>
*<phase>post-clean</phase>*
<goals>
<goal>create-timestamp</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
...
