Answer:
The St. Louis Cardinals have a rich history of success, although their offense this year was awful.
Step-by-step explanation:
The two sentences are expressing contrasting ideas. The first sentence offers a type of compliment to the St. Louis Cardinals. If they have a rich history of success, however, we would not expect the criticism that comes in the second sentence. We are surprised to hear that their offense was awful this year.
To help express such contrast and, at the same time, make the second sentence a subordinate clause, we should choose an appropriate subordinating conjunction. The conjunctions that can help us do that are "though", "even though", and "although", among others. Therefore:
The St. Louis Cardinals have a rich history of success, although their offense this year was awful.
NOTE: We may be tempted to use "but" instead of "although". We need to keep in mind that "but" is a coordinating conjunction, not a subordinating one. That is why it cannot be used here.