猪战（Pig War）是1859年发生于美国和英属北美之间的一场冲突。冲突的焦点为美加边境上圣胡安群岛的归属问题，而引发冲突的直接原因是美国农民射杀了一只加拿大农民的猪，因此此次冲突被称为猪战，又被称为猪剧集、猪和马铃薯战争、圣胡安岛边境争议和西北边境争议。由于没有交火和人员伤亡，这场冲突被视为无血冲突。
1846年6月15日，英美签署俄勒冈条约解决俄勒冈边界争议。条约规定美属俄勒冈地区与英属哥伦比亚地区之间的边界以北纬四十九度划分，一直延伸至温哥华岛与北美大陆之间的海上通道中线，然后南转穿过该通道和胡安·德·富卡海峡，直至太平洋为止。然而，问题在于该海上通道有两条海峡可被视为通道中线，一条是哈罗海峡，位于圣胡安群岛西端，另一条是罗萨里奥海峡，位于圣胡安群岛东端。但在1846年，时人对该地区的地理仍存在一些不确定的地方。当时最常用的地图为1798年出版的乔治·温哥华版本和1845年出版的查尔斯·威克斯版本，两个版本都没有清楚描绘温哥华岛和海湾群岛的东南沿岸海域，因此哈罗海峡在当时尚未为人所知。
1856年，英美设立边界委员会解决双方边界的问题，其中包括乔治亚海峡与胡安·德·富卡海峡之间的水路边界问题。双方在1857年曾就该问题多次会面并交换书信。英方认为根据俄勒冈条约的字眼和条约草拟者的原意，应以罗萨里奥海峡是为界，但美方坚持以哈罗海峡为界。水路边界的争议令双方都宣称拥有圣胡安群岛的主权。
1859年6月15日，即俄勒冈条约生效13年后，条约中的模糊条款引起冲突。美国农民莱曼·卡特莱登陆圣胡安群岛，根据捐赠土地法案宣称拥有这片土地，并射杀了一只闯进他的庄园并啃食马铃薯的大黑猪。这只大黑猪是一个受雇佣于哈德逊湾公司的爱尔兰人查尔斯·格里芬的财产。卡特莱愿意支付10美元赔偿，但格里芬要求100美元。最终英国当局逮捕了卡特莱，随后冲突升级。
When news about the crisis reached Washington and London, officials from both nations were shocked and took action to calm the potentially explosive international incident.
In September, U.S. President James Buchanan sent General Winfield Scott to negotiate with Governor Douglas and resolve the growing crisis. This was in the best interest of the United States, as sectional tensions within the country were increasing, soon to culminate in the Civil War. Scott had calmed two other border crises between the two nations in the late 1830s. He arrived in the San Juans in October and began negotiations with Douglas.
As a result of the negotiations, both sides agreed to retain joint military occupation of the island until a final settlement could be reached, reducing their presence to a token force of no more than 100 men. The "English Camp" was established on the north end of San Juan Island along the shoreline, for ease of supply and access; and the "American Camp" was created on the south end on a high, windswept meadow, suitable for artillery barrages against shipping. Today the Union Jack still flies above the "English Camp", being raised and lowered daily by park rangers, making it one of the few places without diplomatic status where U.S. government employees regularly hoist the flag of another country, though this is for commemoration purposes.
Union Jack at the "English Camp" in San Juan Island National Historical Park During the years of joint military occupation, the small British and American units on San Juan Island had an amicable mutual social life, visiting one another's camps to celebrate their respective national holidays and holding various athletic competitions. Park rangers tell visitors the biggest threat to peace on the island during these years was "the large amounts of alcohol available".
This state of affairs continued for the next 12 years. The dispute was peacefully resolved after more than a decade of confrontation and military bluster, during which time the local British authorities consistently lobbied London to seize back the Puget Sound region entirely, as the Americans were busy elsewhere with the Civil War. In 1866, the Colony of Vancouver Island was merged with the Colony of British Columbia to form an enlarged Colony of British Columbia. In 1871, the enlarged colony joined the newly formed Dominion of Canada. That year, the United Kingdom and the United States signed the Treaty of Washington, which dealt with various differences between the two nations, including border issues involving the newly formed Dominion. Among the results of the treaty was the decision to resolve the San Juan dispute by international arbitration, with German Emperor Wilhelm I chosen to act as arbitrator. Wilhelm referred the issue to a three-man arbitration commission which met in Geneva for nearly a year. On October 21, 1872, the commission decided in favor of the United States. The arbitrator chose the American-preferred marine boundary via Haro Strait, to the west of the islands, over the British preference for Rosario Strait which lay to their east.
British troops evacuate San Juan Island On November 25, 1872, the British withdrew their Royal Marines from the British Camp. The Americans followed by July 1874.
Canadian politicians and public, already angry with the Oregon Treaty, were once again upset that Britain had not looked after their interests, and Canada sought greater autonomy in international affairs.
The Pig War is commemorated in San Juan Island National Historical Park.