Derek was in the army and he had made the mistake of accidentally writing in one of his letters to Meredith a very specific hint as to where he was and she had felt the necessity to follow him. She had been staying with him, hidden in his tent, for a week and he had not yet been able to convince her to go back home. She was only just turned nineteen in September, one month ago and had just started college. He was twenty and had dropped out of college at the end of last year to join the army and fulfill his civil duty. It was the hardest decision that he had ever made, but he knew that it was the right thing to do; even if he was leaving his girlfriend of three years alone, to fend for herself in college, while he was fighting the unknown enemy.
The army hadn't told his crew what or whom they were fighting, they just told them that it was for national security and that it was necessary. The sad part was that it was in California, only two state away from his beloved. He realized a day too late that he had written about how beautiful the golden gate bridge was and consequently Meredith would be able to put her incredibly smart brain to use, and realize that he was in San Francisco. From there, she had wondered every forest in San Francisco, until she had found a campground of large, buff men, somehow managing to evade the camp security. From there, she hid at a local hotel until the early morning, when she snuck into the campsite and found Derek's tent.
The last day that Meredith and Derek had been together, was the day that Derek proposed to her, knowing that if he died, he would want her to know that he loved her enough to want to marry her. After that romantic candlelit dinner, they had slept together for the first time after three years of dating. They were each other's first—and hopefully last. The morning was filled with tearful goodbyes while Meredith drove Derek to the airport, where he was to take a flight to where he would start his training. When they reached security, and Meredith was supposed to leave, she had begged him one last time, trying to remind him of how much she loved him and how she wouldn't be able to go on if he had never come back to him. Derek was only seconds away from running in the opposite direction of the airport, to go back to college with her, when she kissed him with indescribable passion, whispered an 'I love you' and ran off in the opposite direction.