8
Opinion of the Court
the United States and Nebraska a single (and largely overlapping) exception each.
II
We have found that the solicitude for liberal amendment of pleadings animating the Federal Rules of Civil Procedure, Rule 15(a); Foman v. Davis, 371 U. S. 178, 182 (1962), does not suit cases within this Court's original jurisdiction. Ohio v. Kentucky, 410 U. S. 641, 644 (1973); cf. this Court's Rule 17.2. The need for a less complaisant standard follows from our traditional reluctance to exercise original jurisdiction in any but the most serious of circumstances, even where, as in cases between two or more States, our jurisdiction is exclusive. Mississippi v. Louisiana, 506 U. S. 73, 77 (1992) (" 'The model case for invocation of this Court's original jurisdiction is a dispute between States of such seriousness that it would amount to casus belli if the States were fully sovereign,' " quoting Texas v. New Mexico, 462 U. S. 554, 571, n. 18 (1983)); New York v. New Jersey, 256 U. S. 296, 309 (1921) ("Before this court can be moved to exercise its extraordinary power under the Constitution to control the conduct of one State at the suit of another, the threatened invasion of rights must be of serious magnitude and it must be established by clear and convincing evidence"). Our requirement that leave be obtained before a complaint may be filed in an original action, see this Court's Rule 17.3, serves an important gatekeeping function, and proposed pleading amendments must be scrutinized closely in the first instance to see whether they would take the litigation beyond what we reasonably anticipated when we granted leave to file the initial pleadings. See Ohio v. Kentucky, supra, at 644.
Accordingly, an understanding of the scope of this litigation as envisioned under the initial pleadings is the critical first step in our consideration of the motions to amend. We have, in fact, already discussed the breadth of the current litigation at some length in reviewing the Special Master's First and Second Interim Reports, Nebraska II, 507 U. S.,
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