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2002: President Bush signed the Homeland Security Act, and it officially became a department.
Literal
2002-year: Homeland-Security Department establishment-law [on-に] President Bush [subject-が] sign-and officially department becomes.
Historical-timeline entry style, the kind of terse, colon-framed factual listing you'd see in a chronological overview. サインする ('to sign') is a loanword verb — the kana サイン + する construction is how Japanese borrows English verbs. The written-style connective form サインし chains it to the next clause — more formal and compressed than サインして. The present-tense なる ('becomes') is used as a 'historical present,' common in timeline narration where the temporal frame is already set by the year at the start. 国土安全保障省 is the Japanese rendering of the US Department of Homeland Security.