Learn Italian: Ci si allea per vincere, basta che tu sia d'accordo. - People form alliances to win, provided that you agree.

The phrase basta che (provided that / as long as) establishes a condition that must be met. Because it introduces a conditional requirement, it triggers the subjunctive mood. Therefore, we use sia, the present subjunctive of essere.

Why is it ci si allea rather than just si allea?

This is the impersonal form of a reflexive verb. The verb is allearsi (to ally oneself). To make it impersonal ('one allies' or 'people form alliances'), you would normally add the impersonal pronoun si. But putting two si pronouns together (si si) sounds awkward in Italian. To avoid this, the first si changes to ci, resulting in ci si allea.

Why is the subject pronoun tu included here?

In the present subjunctive, the conjugations for io, tu, and lui/lei are exactly the same. For the verb essere, all three forms are sia. The pronoun tu is explicitly included here so it is clear that 'you' are the one who needs to agree, rather than someone else.

Could I use another phrase instead of basta che?