If military servicemen are out of the country during an election how can they participate in the voting process?

Answer 1

A+ = They can fill in and return an absentee ballot.

Answer 2

In the US, the Uniformed and Overseas Citizens Absentee Voting Act tries to deal with this problem. This requires that states allow uniformed servicemen and citizens living overseas to register and cast absentee ballots in Federal elections. It includes a special Federal Write-in Absentee Ballot that can be used by a voter if they apply for an absentee ballot and it does not arrive in time. This provision is important for people in areas where postal service is bad or for people such as soldiers who might have to move from place to place faster than mail can catch up with them. The 50 states have a variety of rules, but in many states, UOCAVA ballots may be returned by e-mail or FAX. Congress has the constitutional authority to impose uniform rules on Federal elections, but it has never done so.

The answer for other countries varies. In some countries, those who are out of the country may sign over a proxy to someone they trust to cast their vote. Some countries have experimented with internet voting. The quality of solutions offered by different countries varies from awful to mediocre. It is really hard to achieve excellence for citizens who are out of the border of the country or on the move at the time of an election without creating gaping vulnerabilities in the election system.