U.S. Defense Secretary Jim Mattis on Wednesday formally returned church bells to the Philippines that were taken as war trophies over a century ago following gruesome clashes, seeking to close a contentious chapter in the two allies' shared history.

The decision to return the "Bells of Balangiga" to the Philippines ends a decades-long quest by Manila, including by President Rodrigo Duterte, and is expected to bolster U.S.-Philippines' relations.

But it has upset some U.S. veterans and Wyoming's delegation to the U.S. Congress, which uniformly opposed returning bells that were a memorial to the 45 U.S. soldiers who were killed during a surprise attack on Sept. 28, 1901, in the central town of Balangiga.