Why might the USSR have wanted to control the Dardanelles? Why Might Truman have found this threatening?

Russian foreign policy, even under the Tsars, was centred around a warm water port, which they have in the Crimea with Odessa etc. However that still means sailing through the Dardanelles. As that was under Turkish control it was a potential threat to them should the Turks want to close it to Russian shipping. Keeping Turkey pro-West was important, not just for this reason, but it also gave NATO/the US an ally in the Middle East and one that was close to the USSR.