Generally speaking, it is easy to commence a litigation in the United States because pleading requirements are not strict. That is, a plaintiff can usually file a lawsuit without including too much detail and without including evidence in the complaint. The additional details and evidence can come later. These same lax rules also apply to counterclaims and defenses.
Rule 8 of the Federal Rules of Civil Procedure imposes three requirements on a complaint: a short and plain statement of subject matter jurisdiction, a short and plain statement showing that the plaintiff is entitled to relief, and a demand. Rule 9 requires some additional detail if a complaint or counterclaim alleges fraud (or a defense relates to a mistake).
Some people criticize lax pleading requirements on grounds that it harms defendants who must pay money to defend or to settle meritless claims. Congress enacted the Private Securities Litigation Reform Act (the PSLRA) in 1995 to curb purportedly meritless securities fraud litigation. The PSLRA makes it more difficult for plaintiffs to commence the lawsuit by requiring additional details in the complaint. Recently, interest groups in the United States and members of Congress have worked to enact stricter pleading requirements and other reforms related to patent troll litigation.
You might want to look at some videos I uploaded regarding pleading requirements in the federal courts.