Dish Network(s dish) may have won over Clearwire(s clwr) board, but Sprint(s s) isn't giving up on Clearwire without a legal fight. Sprint filed a lawsuit on Monday, asking a Delaware state court to stop Dish's investment in Clearwire.

Sprint has already claimed that Dish's offer to take a 25 percent or greater stake in Clearwire is illegal under the corporate laws of Delaware, where Clearwire is incorporated. It now seems prepared to back up its claims before a judge.

Sprint is still Clearwire's largest stakeholder, even though it doesn't have direct control of the company. Sprint is claiming that Clearwire can't accept an offer from Dish without its approval – approval it's not prepared to give. Sprint also claimed that Dish is demanding veto powers and board seats that Clearwire has no right to give due to its equity holder's agreement.

Sprint has been trying to buy up control of Clearwire since December, but it found thwarted by Dish at every turn. Clearwire originally backed Sprint's bid, but in a surprising turn last week it switched allegiances.