Complete the galactic commander's disappointed diary entry by choosing the correct verb phrase.

We ______ Earth on Tuesday, but the Supreme Leader called it off because of a mild drizzle in London.

The correct answer is were going to invade.

We use was/were going to to talk about a plan or intention made in the past that ultimately did not happen (an unfulfilled plan). While "would" is also used for the future in the past, it is generally not used for personal intentions or unfulfilled plans in independent clauses like this one.