The Turkish Parliament on Tuesday extended the military's mandate to launch cross-border operations in Syria and Iraq by two more years.
The motion was first approved in 2013 to support the international campaign against ISIS, and has since been renewed annually.
But this marked the first time that the motion was extended by two years, giving President Recep Tayyip Erdogan a longer mandate for campaigns against Kurdish militias in the restive region.
It also marked the first time the main opposition Republican People's Party (CHP) party voted against the measure, setting it on a more isolationist course ahead of a general election due by June 2023.
"You don't tell us what it's about," CHP leader Kemal Kilicdaroglu said of Mr Erdogan in an address to his party members in Parliament.
"You say for two years and tell us to vote for it. Why?"
The CHP voted against sending Turkish forces to Iraq in 2003, but had otherwise backed Mr Erdogan in his international campaigns.
The new motion allows the military to carry out cross-border operations against ISIS and other groups considered by Ankara to be terrorist organisations.
"The risks and threats to national security posed by ongoing conflicts in regions near Turkey's southern border are continuing to increase," the motion presented to Parliament by Mr Erdogan's ruling AKP says.
This month, he said Turkey was preparing to intensify operations in Syria, where its forces came under attack from a Kurdish militia group supported by Washington in the fight against ISIS.
Turkey and its proxies have seized control of territory inside Syria over four military operations since 2016, focusing heavily on Kurdish militias.
The militants also use their hideouts in northern Iraq as a springboard for attacks on Turkish soil. The Turkish army often bombs their bases in the mountainous regions.
Ankara says it uses its right under international law to self-defence, although the operations cause strains in ties with Baghdad.