If you need to run your tests in a new JVM process you can use the forkMode option to start a new JVM process once for all your tests, or start a new JVM process for each of your tests. You can also set any arbitrary options like -enableassertions or any other JVM options. Here's an example of what this might look like:
<project>
[...]
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<configuration>
<forkMode>pertest</forkMode>
<argLine>-enableassertions</argLine>
</configuration>
</plugin>
</plugins>
</build>
[...]
</project>
Note: You do not need to manually enable assertions if you are using them in your unit tests - Surefire enables them on your test classes automatically under JDK 1.4+.
The default setting is once. It can also be set to never to run in process for a small performance improvement.