During war, troops coming onshore into enemy territory sometimes set up a beachhead, a temporary line of defense they hold until more soldiers arrive.
The World War II term beachhead gets its name from the fact that seagoing forces have landed on a beach. It's also influenced by the earlier bridgehead, a defended position at one end of a bridge. As troops on a beachhead wait for reinforcements, they take a position that's safe and secure, from which they can defend themselves and anticipate advancing further. Figuratively, a beachhead can also be the first step you take toward achievement or progress.