As companies brace for the impact of 2016's political shocks and looming elections next year, global appetite for mega-mergers may slow amid the prospect of increased protectionism.

The biggest transactions mostly involved buyers and sellers from different countries: Bayer AG's acquisition of Monsanto Co. in the U.S., China National Chemical Corp.'s bid for Syngenta AG and SoftBank Group Corp.'s purchase of ARM Holdings PLC. Cross-border deals accounted for more than half of acquisitions larger than $25 billion announced this year, according to data compiled by Bloomberg. They also made up nearly half of the $3 trillion in total announced deals, the data show.

But the sources of international M&A are becoming more inward looking and protective, and it is still unclear how political changes will affect businesses. Chinese firms are facing regulatory scrutiny abroad, with growing opposition in the U.S. and Europe, and restrictions on mega-deals at home.