"A condition of a policy requiring the cooperation and assistance of the assured in opposing a claim or an action lodged against him by an injured person is material to the risk and of the utmost importance in a practical sense. Without such cooperation and assistance the insurer is severely handicapped and may in some instances be absolutely precluded from advancing any defense." (Valladao v. Fireman's Fund Indem. Co. (1939) 13 Cal. 2d 322, 328-329 89 P.2d 643.) "Such provisions 'enable the insurer to possess itself of all knowledge, and all information as to other sources and means of knowledge, in regard to facts, material to its rights, to enable it to decide upon its obligations, and to protect itself against false claims.' " (Wood v. Allstate Ins. Co. (7th Cir. 1994) 21 F.3d 741, 745, quoting Claflin v. Commonwealth Insurance Co. (1884) 110 U.S. 81, 94-95 3 S. Ct. 507, 514-515, 28 L. Ed. 76.) Where an insured violates a cooperation clause, the insurer's performance is excused if its ability to provide a defense has been substantially prejudiced. (Campbell v. Allstate Ins. Co. (1963) 60 Cal. 2d 303, 305-307 32 Cal. Rptr. 827, 384 P.2d 155; Northwestern Title Security Co. v. Flack (1970) 6 Cal. App. 3d 134, 140-142 85 Cal. Rptr. 693.)