The United States will also start construction on a second site in Poland on Friday that is due to be ready in 2018, giving NATO a permanent, round-the-clock shield in addition to radars and ships already in the Mediterranean. The readying of the shield also comes as NATO prepares a new deterrent in Poland and the Baltics, following Russia's 2014 annexation of Crimea. In response, Russia is reinforcing its western and southern flanks with three new divisions. The shield relies on radars to detect a ballistic missile launch into space. Tracking sensors then measure the rocket's trajectory and intercept and destroy it in space, before it re-enters the earth's atmosphere. The interceptors can be fired from ships or ground sites. The Russian ambassador to Denmark warned a year ago that Danish warships would become targets for Russian nuclear missiles if Denmark joined the shield project by installing radars on its vessels. Denmark is upgrading at least one frigate to house a ballistic