If we live in a country where abortion is legal we have an obligation to try to change the law. That is not the final solution to the problem. We need to preach the gospel to change hearts but while we are doing this we should also try to save the lives of the unborn.
Romans 13:1 says, "There is no authority except from God, and those that exist have been instituted by God." We are free to get involved in any institution that God has established.
In the Old Testament God often commanded the Israelites to wage war against their enemies. Since our governments were established by God there can be nothing wrong with a Christian serving his country as a soldier or a law enforcement officer.
Turn to Christ and join us in the New Covenant. :)