The Space Shuttle was designed as a carrier for a First-Strike-Weapon

At the end of the 1960's and the beginning of the 1970's the United States began new research in the use of spacecraft for the destruction of military targets in and from space. In the late 1960's development began at Lawrence Livermore Laboratory of a space-based nuclear weapon-pumped laser. This was originally envisioned as a fearsome weapon, consisting of several dozen independently aimed lasing rods arranged around the bomb. When the bomb exploded, a large percentage of its force would be conducted down the lasing rods toward the targets at which they pointed (in the microsecond before the rods themselves vaporised).

At the same time the Air Force and NASA were studying reusable space shuttles. A single shuttle payload bay of such weapons had the potential of destroying the entire Soviet ICBM force - not just in launch phase but in a first strike, frying them right through the silo covers. One of the most heavily classified projects of the time, it still came to the attention of Soviet intelligence.

During this same period NASA was struggling to justify a post-Apollo space program. The Nixon administration decided that the USAF shuttle project would be dropped, and their requirements incorporated into the NASA design. One of these requirements was a mission involving a launch into polar orbit from Vandenberg Air Force base, release of unspecified payloads into orbit, and return to Vandenberg after a single orbit of the Earth. This requirement forced NASA to drop their preferred straight-wing design for a heavier double-delta wing that had the necessary cross range.

Start from Vandenberg. Fly over the (north-)pole. Release a weapon (and detonate it over the Soviet Union). Return to Vandenberg. Rinse and repeat if necessary.

But in my view, this "First Strike" capability of the Space Shuttle was more a selling point than reality, just like the supposed ease and frequency of the Space Shuttle launches. But to the Soviet leadership, it seemed like a real threat.

The Soviet leadership saw their worst fears confirmed. This was a modern version of the first-strike multiple-warhead UR-500 and N1 super heavy rockets which they had developed but then abandoned in the early 1960's. 

The reaction to the Space Shuttle was the Buran shuttle, which was supposedly very expensive.

It seems that two things broke the back of the Soviet Union: The Afghan War and the Buran project. Both were a reaction to the US – how ironic.