"rival forces" is correct and usable in written English.
You can use the term to refer to two groups of people with opposing interests or goals. For example: "The rival forces in the conflict are now at a standstill."
America and Iran have backed rival forces in the Syrian conflict.
The United States and Russia have backed rival forces during the seven-year civil war.
In the 1980s Walesa and Jaruzelski were leaders of rival forces in Poland.
They may also make the costs of mobilizing higher by numerous strategies, depending upon the relationship between the rival forces.
A power struggle between rival forces of Turkey's Islamist movement is threatening the rule of law itself.
In that case, it took an onslaught by rival forces and airstrikes by the United Nations and France before Mr. Gbagbo surrendered last year.
Part of the problem is that both Saleh and his former deputy, Hadi, are divisive figures unable to bring together the country's many rival forces.
Meanwhile, the devastating effects of civil war continue in Afghanistan's north, where rival forces have been battling for supply routes ahead of winter weather.
During four days of fighting between the rival forces, artillery rounds and gunfire hit two UN bases, killing two Chinese peacekeepers.
Past close encounters between the United States and Russia in Syria have been resolved through the mechanism of "deconfliction" agreements, which outline where the rival forces may operate.
The shelling over Sanaa has terrified residents and emptied the streets, already pockmarked by ground battles between rival forces based in different corners of the capital.
