how did humanity fight the kaijus before they had jaegers

Before Jaegers existed, humanity mostly fought the Kaiju with conventional military force : tanks, aircraft, missiles, naval guns, and whatever heavy weapons could be brought to bear. In the Pacific Rim story, that approach failed badly because the Kaiju were too large, too tough, and dangerously toxic, so cities often took the brunt of the damage before a kill was even possible.

What that looked like

• Governments and militaries tried to stop each attack with everything they had.
• Those attacks usually delayed the Kaiju rather than reliably killing them.
• The Kaiju’s toxic blood made close-range fighting even worse, turning battles into disasters for nearby civilians.

Why it stopped working

The key problem was scale. Regular weapons were built for planes, ships, and armor, not skyscraper-sized monsters, so humanity could injure a Kaiju but not consistently finish one before massive destruction spread. After enough losses, the world shifted to building Jaegers as a specialized answer to a problem normal armies could not solve.

In-universe takeaway

So the short version is: before Jaegers, humanity fought with the military and kept losing. Jaegers were created because that older strategy was proving too costly in lives and infrastructure to keep using.

TL;DR: humanity used conventional weapons first, but Kaiju were too big, too resilient, and too toxic, so the world had to invent Jaegers.