Warning: SPOILERS for Star Trek: Discovery Season 3, Episode 11 - "Su'Kal"
Star Trek: Discovery revealed the U.S.S. Discovery now has a cloaking device, which indicates a controversial change in the policies of the United Federation of Planets in the 32nd century. Since arriving in 3189, Commander Michael Burnham (Sonequa Martin-Green) and the crew of the Discovery were quick to learn the distant future is very different from the 23rd century they left behind. The galaxy has radically changed for the worse thanks to the dilithium catastrophe called The Burn, and both Starfleet and the Federation are a fraction of what they once were. But does Discovery have a cloaking device because it's a special case or has there been a drastic reversal of the Federation's golden ideals?
In Star Trek: The Original Series season 3's "The Enterprise Incident", the Federation ordered Captain James T. Kirk (William Shatner) and Spock (Leonard Nimoy) to steal one of the Romulans' cloaking devices, which they managed successfully. But Starfleet didn't equip their ships with cloaks because of the Federation's principles. The Federation is a symbol of peace and democracy and, simply put, Starfleet ships don't go sneaking around the galaxy. Some, like Star Trek: The Next Generation's Admiral Pressman (Terry O'Quinn), felt this placed Starfleet at a distinct disadvantage against enemies like the Romulans and Klingons that used cloaking technology, and he disastrously tried to develop a cloak for Starfleet aboard the U.S.S. Pegasus, but the Federation's lofty ideals continued to be upheld.
On Star Trek: Deep Space Nine, the U.S.S. Defiant was equipped with a cloaking device borrowed from Romulans, which was also controversial. Initially, the Defiant was only supposed to use the cloak when it traveled to the Gamma Quadrant, which was ruled by the Dominion, and there was even a Romulan officer aboard to oversee proper use of the cloak. However, that plot device was soon dropped and Captain Benjamin Sisko (Avery Brooks) would use the Defiant's cloak whenever it was convenient. Still, the Defiant was only one ship with a cloaking device, but Star Trek: Discovery begs the question of whether all Starfleet ships in the 32nd century use a cloak.
In Star Trek: Discovery season 3, episode 11, "Su'Kal", Acting Captain Sylvia Tilly (Mary Wiseman) called for Discovery to activate its cloaking device when the flagship of the Emerald Chain confronted the Starfleet vessel outside of the Verubin Nebula. Tilly noted that the cloaking device was part of the series of upgrades of 32nd-century technology the thousand-year-old starship received when it was recommissioned into the U.S.S. Discovery-A. Of course, the Emerald Chain's ship, the Viridian, also has a cloaking device, but that's not surprising since the Chain is a criminal syndicate.
However, since Discovery received a cloaking device as part of its retrofit, the question is, do the rest of Starfleet's ships also now use cloaks as a rule? Granted, the Federation and Starfleet are drastically underpowered in the century since The Burn happened, and the galaxy has only become more dangerous. It would be understandable if the Federation had to amend its policy regarding the use of cloaking devices as a defensive measure. In the 24th century, the Federation was bound by the Treaty of Algeron it signed with the Romulans where it pledged not to use cloaking technology, but that was 800 years ago in an Alpha Quadrant existing under vastly different circumstances.
Yet it's also likely that Discovery is the only Starfleet ship allowed to have a cloaking device because of special circumstances: its spore drive. In a galaxy lacking dilithium for warp technology, Discovery's spore drive is an irreplaceable advantage that also makes it a target for enemies like the Emerald Chain in Star Trek: Discovery. So, the Discovery could have been granted a cloaking device as an extra defensive measure to protect itself and its spore drive. Indeed, the spore drive was the very reason Osyraa (Janet Kidder) seized the U.S.S. Discovery in "Su'Kal" but, unfortunately, not even a cloaking device was able to stop the Emerald Chain.
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