The Flint Assembly plant has been an integral part of General Motors for more than 70 years, turning out more than 13 million vehicles and providing the livelihood of generations of workers. But on Monday, the factory’s output of Chevrolet and GMC trucks came to a halt.

That’s because the United Auto Workers, whose ranks include some 4,800 hourly workers at the Flint plant, went on strike against G.M. for the first time in 12 years. Much has changed since then: G.M. went through bankruptcy, shed brands, pared its United States work force and made an emphatic return to profitability.

Though workers have drawn annual profit-sharing checks, the union says G.M. has done too little to repay employees for past concessions and to reinforce their job security. Negotiations resumed on Monday. But the sights and sounds outside the Flint plant signaled confrontati