Amendments to Pleadings under the Federal Rules
Under the Federal Rules of Civil Procedure, parties commence litigation by filing their initial pleadings---the complaint and the answer. The plaintiff starts the action by filing the complaint (Fed. R. Civ. P. 3) and serving it on the defendant. In most cases, the defendant responds by filing an answer. Fed. R. Civ. P. 12(a)(1)(A). These pleadings set forth the basic positions of the parties about the dispute, the claims and defenses they hope to establish at trial.
Naturally, when the parties file the original complaint and answer, their understanding of their claims and defenses will be incomplete. They will certainly have done some investigation, and given serious thought to their legal theories and positions on the disputed facts. Indeed, Rule 11 requires them to do so. See Fed. R. Civ. P. 11(b)(2), (3). But at the time the complaint and answer are drafted, much of the detailed development of the case lies ahead. Parties are not entitled to engage in discovery---the process of court-enforced disclosure of information by other parties and witnesses---until the case has been filed. And, given human nature, the intense focus of many litigators is not on embryonic cases that are at the pleading stage, but mature ones that are about to go to trial.