UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (date of earliest event reported): June 20, 2013

Starburst II, Inc.

(Exact Name of Registrant as Specified in its Charter)

11501 Outlook Street 4th Floor

Overland Park, Kansas 66211

(Address of Principal Executive Offices, Including Zip Code)

(617) 928-9300

(Registrant’s Telephone Number, Including Area Code)

N/A

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Item 8.01 Other Events.

On June 20, 2013, Sprint Nextel Corporation (“Sprint”) and Clearwire Corporation (“Clearwire”) announced that they had agreed to amend their merger agreement providing for the acquisition by Sprint of the approximately 50 percent stake in Clearwire that it does not currently own. Sprint also announced that it had entered into voting and sale agreements with a group of significant Clearwire stockholders pursuant to which such stockholders agreed to vote their shares in support of the transaction, and pursuant to which Sprint and such stockholders agreed to certain stock purchase arrangements triggered upon any termination of the merger agreement. As part of these agreements, Sprint and such stockholders also entered into mutual releases in favor of each other relating to the transaction. Starburst II, Inc. (the “Company”) agreed to become a party to these agreements for the limited purpose of joining in providing to such stockholders, and receiving the benefit from such stockholders of, these releases. A copy of the joint press release of Sprint and Clearwire announcing these arrangements is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

Cautionary Statement Regarding Forward Looking Statements

This document includes “forward-looking statements” within the meaning of the securities laws. The words “may,” “could,” “should,” “estimate,” “project,” “forecast,” “intend,” “expect,” “anticipate,” “believe,” “target,” “plan,” “providing guidance” and similar expressions are intended to identify information that is not historical in nature.

This document contains forward-looking statements relating to the proposed transactions between Sprint Nextel Corporation (“Sprint”) and SoftBank Corp. (“SoftBank”) and its group companies, including Starburst II, Inc. (“Starburst II”), and the proposed acquisition by Sprint of Clearwire Corporation (“Clearwire”). All statements, other than historical facts, including, but not limited to: statements regarding the expected timing of the closing of the transactions; the ability of the parties to complete the transactions considering the various closing conditions; the expected benefits of the transactions such as improved operations, enhanced revenues and cash flow, growth potential, market profile and financial strength; the competitive ability and position of SoftBank or Sprint; and any assumptions underlying any of the foregoing, are forward-looking statements. Such statements are based upon current plans, estimates and expectations that are subject to risks, uncertainties and assumptions. The inclusion of such statements should not be regarded as a representation that such plans, estimates or expectations will be achieved. You should not place undue reliance on such statements. Important factors that could cause actual results to differ materially from such plans, estimates or expectations include, among others, that (1) there may be a material adverse change of SoftBank; (2) the proposed financing may involve unexpected costs, liabilities or delays or may not be completed on terms acceptable to SoftBank, if at all; and (3) other factors as detailed from time to time in Sprint’s, Starburst II’s and Clearwire’s filings with the Securities and Exchange Commission (“SEC”), including Sprint’s and Clearwire’s Annual Reports on Form 10-K for the year ended December 31, 2012 and Quarterly Reports on Form 10-Q for the quarter ended March 31, 2013, and other factors that are set forth in the proxy statement/prospectus contained in Starburst II’s Registration Statement on Form S-4, which was declared effective by the SEC on May 1, 2013 and in other materials that will be filed by Sprint, Starburst II and Clearwire in connection with the transactions, which will be available on the SEC’s web site (www.sec.gov). There can be no assurance that the transactions will be completed, or if completed, that such transactions will close within the anticipated time period or that the expected benefits of such transactions will be realized.

All forward-looking statements contained in this document and the documents referenced herein are made only as of the date of the document in which they are contained, and none of Sprint, SoftBank or Starburst II undertakes any obligation to update any forward-looking statement to reflect events or circumstances after the date on which the statement is made or to reflect the occurrence of unanticipated events except as required by law. Readers are cautioned not to place undue reliance on any of these forward-looking statements.

(d) Exhibits

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

EXHIBIT INDEX